I've seen this on several threads here and not properly addressed.
On opening current projects I'm finding clips are randomly shifting their start points, within the timeline placeholder. Ie. The timeline is all correct but the source clips appear to have moved reference, and therefore are out of sync with the full sequence. I tried matching frame rates in Media Encoder before and this seemed to have solved it, until now. I've rolled back to a previous version and still seem to have the issue.
Some clips even display the greyed out pattern, showing that the source has so much its in a blank area.
A very very frustrating bug as I now have to treble check everything.