My Albums in Photos no longer show filenames
Filenames show under photographs when in Library, but have disappeared in all of My Albums.
Do still have enabled the checkmark in the View menu in Photos?
View > Metadata > Titles?
Then you should be seeing the filenames below the thumbnails, unless the photo has a title, then the title will be displayed instead. But you have to use a view, where the photos are not tiled.
And are you using the option to view the thumbnails in the correct aspect ratio and not as squares? And set the zoom slider to see large thumbnails.
See: How to view the titles or filenames below the thumbnails in Photos 5.0 - Apple Community
How to switch between the tiles and the correct aspect ratio to see the thumbnails without cropping:
How to enable the titles below the thumbnails in the All Photos view and the albums: