This is a strange one I've never seen in my nearly 20 years of editing.
When I received a batch of footage with sync timelines, I always duplicate them and create "markup" sequences with my initials where I leave color coded markers to help myself navigate the project more fluently.
Altogether in the project at-hand, I created six duplicated sequences with markers. After leaving the project for a week, I opened it today and all of my marked sequences had vanished from the project but one. I searched "sequence" in the project browser just to make sure I hadn't somehow moved all six sequences to a random folder. I then thought, "no biggie, I have autosave set to save every 5 minutes, so I'll just go back to a previous iteration."
When I opened the autosave project, the sequences were missing from it as well. I then went through two weeks of autosaves. The sequences are missing from all of the project files as if I never created them. The thing is, I did. I used them throughout the entire edit to arrive at a final cut. This is a really alarming bug and I've seen quite a few in my time.