It may not be an Acidized file. I've used thousands of them over the years and Ctrl+L turns groove clip looping on and off every time. Of course I don't know if FL locks the format or does something else that's not a standard acidized file.
 
You may be trying to treat a REX file like an Acidized file. They are not the same thing. SONAR will import a REX file with its transient markers intact because they are an inherent part of the REX file format. It will also look like an Acidized file in SONAR because you can roll it out, loop it, etc.
 
However, you cannot turn a REX file's transient markers on and off as you can with an Acidized file, any more than you can have the REX file follow pitch markers. The file format doesn't allow either option.

Anderton
It may not be an Acidized file. I've used thousands of them over the years and Ctrl+L turns groove clip looping on and off every time. Of course I don't know if FL locks the format or does something else that's not a standard acidized file.
 
You may be trying to treat a REX file like an Acidized file. They are not the same thing. SONAR will import a REX file with its transient markers intact because they are an inherent part of the REX file format. It will also look like an Acidized file in SONAR because you can roll it out, loop it, etc.
 
However, you cannot turn a REX file's transient markers on and off as you can with an Acidized file, any more than you can have the REX file follow pitch markers. The file format doesn't allow either option.

I think I just figured out what's causing the 'lock'.
The audio file was rendered at 32bit float.
Maybe SONAR isn't fond of 32bit audio files.
 
 If anyone wishes to try it out, here's a link to the loop I made...
https://www.dropbox.com/s...it%20Float%29.wav?dl=0

Samuel540

Anderton
It may not be an Acidized file. I've used thousands of them over the years and Ctrl+L turns groove clip looping on and off every time. Of course I don't know if FL locks the format or does something else that's not a standard acidized file.
 
You may be trying to treat a REX file like an Acidized file. They are not the same thing. SONAR will import a REX file with its transient markers intact because they are an inherent part of the REX file format. It will also look like an Acidized file in SONAR because you can roll it out, loop it, etc.
 
However, you cannot turn a REX file's transient markers on and off as you can with an Acidized file, any more than you can have the REX file follow pitch markers. The file format doesn't allow either option.

I think I just figured out what's causing the 'lock'.
The audio file was rendered at 32bit float.
Maybe SONAR isn't fond of 32bit audio files.
 
 If anyone wishes to try it out, here's a link to the loop I made...
https://www.dropbox.com/s...it%20Float%29.wav?dl=0

I just downloaded the file, and imported it into SONAR. Then I bounced to clip (I have Shift+Ctrl+C as my shortcut). Groove looping turned off.
Ctrl+L to turn on or off again.

 
 

The transient markers for the downloaded loop show as orange in the loop construction window, which is unlike any other Acidized file I've loaded. So I used a hex viewer to compare the downloaded file to a "known to be Acidized and you can turn it on and off" file. There were some differences in the header, although I don't know enough about the topic to determine what kind of difference that makes. 
 
If you save the imported loop from the Loop Construction window, it will do whatever conversions are necessary to create a compatible Acidized file. I have not tried the file with Acid to see if it recognizes the format or not, but that would be interesting.

 
I prefer not to use the Bounce to clip(s) process especially in this case where the tempo of the audio file was not accurate in SONAR and resulted in a crackling sound throughout the clip. The 'crackling' remains present even after I used Bounce to clip(s).
 
Hopefully, the developers catches up with this.

Samuel540
 
I prefer not to use the Bounce to clip(s) process especially in this case where the tempo of the audio file was not accurate in SONAR and resulted in a crackling sound throughout the clip. The 'crackling' remains present even after I used Bounce to clip(s).

I experienced the same crackling sound when using Bounce to clip(s) with your file.
 
Have you tried @Anderton's suggestion?  [quoted below]
After following the suggestion, I was able to enable/disable Looping and the file did not have the crackling sound.
[EDIT] The resulting file also had the correct tempo of 118 BPM.
 

Anderton
If you save the imported loop from the Loop Construction window, it will do whatever conversions are necessary to create a compatible Acidized file.

 
-- Ron

SquireBum
Have you tried @Anderton's suggestion?  [quoted below]
After following the suggestion, I was able to enable/disable Looping and the file did not have the crackling sound.
[EDIT] The resulting file also had the correct tempo of 118 BPM.
 

Anderton
If you save the imported loop from the Loop Construction window, it will do whatever conversions are necessary to create a compatible Acidized file.

-- Ron

Yeah, there's something weird about the file format but it is a "mostly Acidized" file. So if you save it within SONAR's Loop Construction window, that makes whatever changes are necessary. You do not have to bounce the clip to itself.
 
It may relate to the 32-bit floating point, I don't know. All Acidized files I've seen are 16- or 24-bit. You might also want to check if FL Studio has any export options that can be set to solve the issue. As I mentioned, the orange transient markers are definitely something I haven't seen before in SONAR.

SquireBum
Have you tried @Anderton's suggestion?  [quoted below]
After following the suggestion, I was able to enable/disable Looping and the file did not have the crackling sound.
[EDIT] The resulting file also had the correct tempo of 118 BPM.

Anderton
If you save the imported loop from the Loop Construction window, it will do whatever conversions are necessary to create a compatible Acidized file.

 
I did but there was still some muddiness in the sound. It wasn't as distinct as the crackling noise but it was still unpleasant.
My guess is that the development team never got around to run tests for the 32-bit float format on loops/ACID files in SONAR... 

"All Acidized files I've seen are 16- or 24-bit." Quote Craig Anderton.
You clearly must have missed this to still be referring to this as a developer issue. 
Go back and export your file from FL Studio at a lower bit rate and report back so we can all learn if that is the fix for your issue.
 

Thatsastrat
"All Acidized files I've seen are 16- or 24-bit." Quote Craig Anderton.
You clearly must have missed this to still be referring to this as a developer issue. 
Go back and export your file from FL Studio at a lower bit rate and report back so we can all learn if that is the fix for your issue.
 

Well, I've already settled it. It's definitely the bit rate.
I've experienced this issue a couple times before in the past but never knew why. I've recently ran into the problem again so, I decided to experiment with it. The confusing and very frustrating part of it was trying to understand why the audio file was being imported as a loop in SONAR, but when dragged into FL Studio and Ableton Live, the audio file was imported as normal. No weird, sped-up tempos etc.. Not to mention, you cannot disable the looping or change clip tempos.