iOS 18.7: Plex App, downloads of various series episodes never play

Downloads complete, though excruciatingly slow (32mbps) over a 5G WiFi lan connection to the server which otherwise gets a roaring speed of well over 500mbps in most cases, refuse to play after completing. They just sit there with the loading animation, forever. Never play, even if they're supposedly downloaded locally. This has been going on for at least a month so it was before the 18.6 update to iOS and whatever updates might have happened between now and the last time this worked for me, which might have been in July. Any ideas what is happening? Will this post be immediately deleted with no explanation because asking questions on the internet is now forbidden?