Good to know.
I tested this with macOS 13.5.1 in .mp4, .m4v and .mov movies with the following tag for Description which works in QuickTime Player and Photos (I inserted the description to movies with GraphicConverter 12 and then imported to Photos, if that matters):
Keys:Description:himenaeos vehicula tempus etiam primis tempor sagittis semper eget parturient
I.e. the description has 10 words:
himenaeos vehicula tempus etiam primis tempor sagittis semper eget parturient
The 8th word "semper" is the last word Photos finds as an individual word in all those movies. I wonder if the limit is in characters or words.
I also tested in Finder with Spotlight with those individual words:
Spotlight finds all 10 of them. BUT JUST in .mp4 and .mov.
Spotlight does not find ANYTHING in .m4v.
But if I just change the .m4v suffix to .mp4 or .mov, THEN Spotlight finds all Descriptions.
This is bad because apps like GraphicConverter use Spotlight for search so GC can not find anything in .m4v either.
https://eclecticlight.co/2023/06/22/what-to-do-when-spotlight-cant-find-it/comment-page-1/?unapproved=87135&moderation-hash=23f44bc4ae22a3d47f3e8188cdbd9c6d#comment-87135
I have reported this to Apple's feedback page:
Feedback - macOS - Apple