I now have some movies that used to play fine on all devices that are no longer playing on multiple Apple TV 4s.
The same movies play fine on iOS, macOS, Roku Premiere+, Win8, WIn10 and using PMP or the web player. It is only the Apple TV 4s that will not play them. They just sit at a black screen with a spinning circle until the screensaver kicks in.
I have pulled some logs but am not sure which ones I should be looking at.
PMS is running the latest version (18.104.22.16800) on my iMac.
ATV4s are running what I think is the latest version (1.6).
Files are stored on a Synology NAS DS1815+.
The files in question are h264 in an mkv container but so are all the other files that play fine.