Clip speed duration in timeline cause glitches issues in exported videos
latest version on Big Sur, Mac mini M1 chip. Have noticed that if you alter the speed of a clip, on export, it glitches, flips clip or image for a frame, disappears for a frame... I had reduced speed on a clip to 76%... it caused me a headache and client was getting irate. as soon as I put the clip to 100% and added it again under a transition, it worked fine... happy client. You also cant export ProRes4444ALPHA without experiencing nightmare issues.
AFAIK Premiere pro does not support export of alpha channels - you need After effects for that.
Similar problem here. M1 Mac Mini. Repeated frames when speed adjusting clips on render. Would actually play a previous frame after a new frame or jump forward a couple of frames resulting in steppy, glitchy playback.
Did solve problem by switching to software render. But this is a serious bug and I was in the same situation - client over the shoulder, people depend on you and I was delayed by about an hour sorting it out as I tried different things (like switching renderers from optical flow back to frame blending etc.)