To: Esther
When I was having the issue it would be present in the following situations:
When mouse-dragging the time slider,
When Keyboard key moving the time slider,
When pressing play (Space) for preview,
In the Produce preview window,
In the final exported animation.
The greatest problem is that these occurances (seemingly) weren't related, that is it could appear in the preview, but not in the exported file or not appear in the preview and appear in the exported file (and exported file preview).
By doing some more testing it seems like I may be getting closer to the root of the problem - it seems like when you click a different overlay it shows the preview of the last moment at which you were editing this overlay until you move the time slider. When you move the time slider the preview frame becomes the overlay without any fade-in effect. And for some reason this behavior can somehow transfer to the export preview and the exported file.
I have recorded a video of this repeatedly happening and will try to attach it as well. Seems to have worked - so you can see dragging the time slider, moving it with keys, playing the preview - most of the time the last viewed frame from that overlay flashes before the fade-in.
I guess the main question is if this is planned behavour - it seems to make sense to show a fully visible overlay when you click it and show the fades only when in motion, but less so when the two mix together.
PS. The Screen recorder is very neat, really helps to be able to show a problem, but it could use lossless compression and it doesn't produce any video when trying to record small regions (like 200x100px or 100x100px). Also sometimes crashes when trying to record a small region.