Difficulty syncing movies from Mac photos to my iPhone

You can search for this filename, if you open the Photos Library in the Finder.

Find your Photos Library in the Finder, then ctrl-click or right click it. Now use the command “show package contents”. In the window that opens go to the folder originals. Now search for the name of the file. Be careful not to modify anything, just look.

Apple is renaming all files internally in the Photos Library with unique names. Photos 5 on Catalina is the first version of Photos that does no longer store our original image files internally unmodified. The Photos.app is showing us the original image files, but it has to look them up in the internal databases. When you sync movies to your iPhone with the Finder, the Finder is seeing just the renamed files.

To make it easier to see the renamed filenames you could get yourself the app PowerPhotos from Fatcatsoftware: https://www.fatcatsoftware.com/powerphotos/

The free trial version will suffice to see the internal filenames. When you select an image from your Photos Library in PowerPhotos, you can see the internal pathnames, for example: