I have a couple users with 365 that have this and nothing in this thread I tried worked.
I put in a ticket with Microsoft and they responded quickly and we retried and attempted a few things through quick assist but nothing resolved it even starting in safe mode. They confirmed this is a known issue, had no more suggestions to try but documented settings and forwarded this to their back end team who is still investigating this.
One workaround that mitigates it slightly is I noticed at least for one user, that it would jump down to emails roughly around 1-13-25 or a bit older. I moved all inbox emails older than 1-15-25 to a new subfolder and the jumping problem (so far) stays with that subfolder and the inbox functions normally so at least working with current emails they won't have to deal with it. It's possible that just sending a smaller chunk that includes all of the "bad" emails to a subfolder would work too.
[Some things tried: none had Grammarly, focused inbox toggled on/off, online office repair, reset views (in GUI and outlook.exe /cleanviews), disabled all add-ins, highlighted all emails and set "clear flags", also tried outlook.exe /cleanips and clicking "clear offline" items for inbox) (deleting the .ost would temporarily fix it until the cache repopulated to a certain point again. )]