I have a film of me trying to land on the CV after a 2 hour mission only to have some guy turn it every time I was on short final.
After the first change of course, they disappear.........but only when you have selected and copied a portion of a much longer film to make a new one.
If the beginning of your selection is anything but 00:00:00, this happens but there are no problems if you're editing off the end.
In the original movie, they don't disappear but it's a very long film.
This might be better in the bugs forum.