Hello Craig, I have created a video to show the issue.
 
https://youtu.be/UO8yHwxj2g8
 
Notice that I am demonstrating this in the rolled back version of 2016.12, so I guess I was mistaken about it being an issue with the update to 2017.01.  It is just such a strange coincidence that I never experienced this before the update and it was fine again when I rolled back;  until I rebooted my machine.  I have to believe I jumped to the wrong conclusion on it being an update issue -- but I still think the behavior is strange and I want to disable it.
 
I have updated the title accordingly.
 

Anderton
Yes, that's certainly true and exactly why more details are needed.

I made a video (see OP or previous post above) which shows the issue.  I did manage to reproduce it in 2016.12.  So I updated back to 2017.01 and you want to know what's crazy?  I can no longer reproduce it.  I am using the same simple project in that video and now I can't make it happen.  This confirms my suspicions that the world has gone mad -- or I have :P
 
But seriously, what you see in the video is what I encountered for the first time ever after I updated to 2017.01.  Then when I rolled back I was able to reproduce it.  Now I've updated again and I can't reproduce it!  I'm a veteran software engineer, and this makes no sense, lol!

There is a point in that video where you seemed to move both clips as a unit before a fade and then the split issue. Not sure if that step needs to be done to reproduce.

mettelus
There is a point in that video where you seemed to move both clips as a unit before a fade and then the split issue. Not sure if that step needs to be done to reproduce.

I keep trying and have spent 5 minutes quickly moving clips this way, that way, crossfading changes, fade changes, back and forth, and now I can't reproduce it.  That moment in the video where both clips move as a unit occurred without me wanting or trying to do it.  It's almost like something weird was going on "lanes wise" (under the covers).
 
The thing is I couldn't avoid this issue before so I was sure that someone else would see it quickly and easily.  Because everything I was doing in that video was happening all the time.  Now it's happening none of the time.  I do a *lot* of clip movement in my work so this really came out of the blue.  There has to be a scientific reason but it's not what I had thought originally (i.e. the update).

your grabbing the clips in the wrong spot mate.
Try doing the exact same thing you are doing but be aware of the clip line (at the top of the clip)
Only grab it (to move) by clicking on the right side of that line. Sonar is thinking you want to grab the selected part underneath.
 
I watched your video and can see where you are grabbing it from. be aware of that clip separation line on the clips header.
 

chuckebaby
your grabbing the clips in the wrong spot mate.
Try doing the exact same thing you are doing but be aware of the clip line (at the top of the clip)
Only grab it (to move) by clicking on the right side of that line. Sonar is thinking you want to grab the selected part underneath.
 
I watched your video and can see where you are grabbing it from. be aware of that clip separation line on the clips header.

Hey chuckebaby, how's it going mate?
 
I appreciate you chiming in.  But I am sure I am doing exactly the same steps now as in that video but cannot reproduce it.  Which is good in a way, because I don't want that issue to occur.  I do want to know what I did that caused it, but I am clicking and moving from the top of the clip, to the left, to the right, just above the line, just below the line, and as The Beatles sang, here, there and everywhere.  
 
I'm normally moving so many clips so rapidly that I take all this for granted.  And then suddenly I encountered this strange overlapping clip syndrome where the overlap would split away.  And I couldn't stop it.  For a while it seemed to be happening on a lot of clips.  Now it's not happening at all.  I know I must be doing something different to have made it happen, but I believe I am following all the same steps and many more.  I will keep trying because it was disconcerting to see it happen last time and I realized a whole bunch of clips had been affected in one session (though luckily I backup each and every session so I could return).

It happened again last night, moving single-note guitar clips (cut from a long take but not bounced-to-clip yet)
Moved two clips so they overlapped, auto crossfade did it's thing, I moved another clip a little, came back and moved the overlapping clips apart and there it was. The crossfaded bit broke off and was left behind.
 
Last time I was in a discussion about this phenomenon was I'm sure six months or more ago, so it's been around some time.

Tried again, cannot reproduce.
 
I have changed no settings and it's the same track and the same clips, unchanged, no bounce-to-clip or anything like that.
 
It looks like it's random.

jpetersen
It happened again last night, moving single-note guitar clips (cut from a long take but not bounced-to-clip yet)
Moved two clips so they overlapped, auto crossfade did it's thing, I moved another clip a little, came back and moved the overlapping clips apart and there it was. The crossfaded bit broke off and was left behind.
 
Last time I was in a discussion about this phenomenon was I'm sure six months or more ago, so it's been around some time.

Well I'm glad I'm not the only one, though that doesn't mean I am happy you experienced the same/similar issue.  It's just a feeling of validation.  And this is just my rain dance :P
 
Seriously, I think I may have a bit more insight to this issue, though nothing definitive.  I opened up the track's lanes (which I never use btw), and saw that both clips are on the same track (nothing unusual there).  Then I began to move the clips around on the same lane, and I was able to very quickly reproduce the "issue".  Maybe this might be expected behavior whilst in take-lanes editing mode, but I found it disconcerting especially as it didn't even do any cross-fading for overlapping clips.  Instead the overlaps were "swallowed" and just seemed to disappear -- and could only be revealed by slip-editing the clips (normally I would just move clips apart and the crossfades would change, but here I was not able to do that the way I expected).
 
I *think* that maybe there is some dual functionality that is at odds with itself.  Meaning that when in track mode there is one type of clip editing behavior, but when in take lanes mode there is a different behavior.  And that somehow the lanes functionality is creeping into the track functionality "randomly".  I keep thinking there must be some reproducible steps when in track edit mode, but I cannot seem to ascertain what it is.  So it feels random.
 
 

VariousArtist
I opened up the track's lanes (which I never use btw), and saw that both clips are on the same track (nothing unusual there).  Then I began to move the clips around on the same lane, and I was able to very quickly reproduce the "issue".  Maybe this might be expected behavior whilst in take-lanes editing mode, but I found it disconcerting especially as it didn't even do any cross-fading for overlapping clips.

 
The "rules" for moving things around in Take Lanes are indeed different compared to moving when no Take Lanes exist.