I noticed this problem recently, and today found that it could be fixed by analysing the files, this can be done one-by-one, but in a case like this it’s best to perform it on the entire library:
Hover over your library on the left side menu, and click the … button to open a sub-menu
Select “Analyse” from that menu to analyse all files in your library.
The resolutions for all your movies should then repopulate in the library.
I don’t know if you’re still having this problem, but even if you aren’t, hopefully it will help someone else.