Plex should default to the first “theatrical” release date. Film festivals usually don’t count.
Did you tell Plex to “prefer embedded meta tags”? The wrong date could be stored inside a mp4/m4v file.
Only if you still use the old deprecated agents (i.e. NOT “Plex Movie”):
Perform only the first 3 steps from this guide: [HowTo] Reset metadata agent preferences to defaults and whack caches
There were a few date-related bugs in PMS 1.26.0 which have been fixed in PMS 1.26.1 Please update to this beta version and re-evaluate.