In my (crude) testing, at 80% retiming, if I space the edit points far enough apart I don't see any issue. At 200% I didn't see the issue, either. Same with rendering or without. To be clear, this is with edit point intervals greater than one second.
For a case with a single < 1 sec. edit point interval:
- At 80%, I see two glitches, one at each edit point where the interval is less than one second. When stepping through, frame by frame, at each edit point there is a repeated frame. You can see the two glitches during normal playback.
- At 200%, stepping through I don't see a glitch at either edit point. While holding the right arrow key down, I don't see a glitch. At normal playback speed, I just see one glitch, though it looks more like frame jutter, or out of order frames, and not duplicated frames as in the 80% case.
FYI, I did not export and look at the resulting outputs.
Your test case used a timeline at 23.976 fps. I used 29.97 fps.
Of course, there are so many variables that could be involved it's hard to really make any sort of definitive statement (for me, at least).
If you have a specific case(s) you'd like me to verify/test, let me know.
And, yes, it's a bit surprising that no one noticed this behavior before. Personally, I apply retiming to single clips, on a limited basis, so I've never run into this. Usually, I may apply audio effects or color effects to a compound clip in my typical usage scenario...