I have multiple movies in each folder. Watch Now and Open Containing Folder both do the same thing, they go to one particular movie in the folder, whatever other movie might be selected.
I can't tell how that file is chosen. It's not the first file in the folder using any criteria - alphabetic, date, unsorted order, position in xml file. But it's always the same file.
A possibly related oddity is that Find Duplicate Movies reports all the movies in one folder as duplicates.