I don't ever remember having this issue before. 
 
I drag a Rex (.rx2) file from the Sonar media area and drop it on a new track at the first measure.  I'm not using the first 4 measures of the rex file, so I grab the handle and slide the start time over to measure 5.  I'm not moving the rex file, just cutting off the first 4 measures.  Now, I want to grab the whole track and slide it left to measure 1, but Sonar won't let me.  I can move the track right to a later measure, but not left to an earlier measure.  It's like Sonar still detects the track as starting on measure 1.  Is this normal behavior?
 
Now, if i right click the rex file and bounce to clip, afterwards I can move the bounced track left or right no problem. 

I'm no expert though I'll venture to say that to the program the clip is still the same length.  The handle to move the start of the clip does not shorten it at all... only applies a mute to the first 4 bars.  They're still there.
 
When you bounce it, you are creating a clip without the first 4 measures.  As a result, you can slide it around as desired.
 

I think you nailed it. The graphic representation is just that - the underlying clip is the same, unless you trim it.

That was my thought as well.  After a little more testing, I found that the same behavior does not occur when working with .wav files, which is mostly what I have been using lately.  I can edit the start of a .wav, then move the clip left or right on the track, with no problem.  

Groove Clips are a special case, and REX files are converted into Groove Clips upon being imported.

Thanks, I didn't know that .rex files where converted to Groove clips.  Good to know.
 

Anderton
Groove Clips are a special case, and REX files are converted into Groove Clips upon being imported.