Home Sharing - nearly all movies are missing

I have an MBP using an external drive for media connected via USB-C. A couple of hundred movies. Everything has been relatively stable (for Home Sharing at least) for several months now. Suddenly, this week, when I try to access shared movies from an iOS device (Apple TV 4, iPhone SE2, iPad Mini 5), only 4 movies are listed.

• All devices are fully up-to-date.
• The 4 movies that do show up are listed as "Protected MPEG Video File" -- but so are most of the other movies in my library. I don't see anything unique about those 4 movie files.
• The entire group of movies are accessible/watchable on the MPB.
• If I connect the iPhone or iPad via USB-C to the MBP, I can access and side-load any of the movies. This isn't an option for the ATV.

I've tried:

• deauthorizing/reauthorizing on the MBP
• disabling/re-enabling Home Sharing on the MBP and iOS devices
• rebooting all devices

All to no avail. I'm not even sure switching to some LAN-based alternative would solve this. As of now, the only way we can watch these movies on the ATV is to play them on the MBP and Airplay them to the ATV.

This constant breaking of Home Sharing is really tiresome.