I tried the “plex dance”, several times : it had no effect on this issue.
Then I tried to deleted my photos library and add a new one… even with a new one, the timeline is messed up !
Not the same mix up than the previous library, but not OK either.
The most recent photos are in the right place, but the more you go back in time in the timeline, the more photos are shown under a wrong date.
It’s been a month since I posted the first message, is anyone from the plex staff even reading this ?