Here's a link to the old thread, which may have more information - https://discussions.apple.com/thread/250811432
And I appreciate the tip to not post files here - as the problem I have is with dvd rips of DVDs I own, perhaps it wouldn't be a good idea to post links to them publicly. If anyone needs to see one, let me know privately.
Some clarification - I like to watch tv shows and movies in the TV app - that's what it's for, after all. And it's convenient to have it all working in one app. And the TV app still works great, as long as the movie content is stuff you bought or rented from apple. However, you used to be able to play files from any source, within reason, including files ripped from DVDs, bought on amazon, privately produced, or whatever, and it would work fine. Basically if it would play in quicktime on your mac it would work in the TV app (and/or whatever it used to be called). Starting with the first beta of iOS 13, third party movies (those not bought or rented from apple) started having this audio/video sync problem. I reported it, and reverted back to iOS 12 for the duration of the beta program. When I installed the official first version of iOS 13, I was amazed that the problem was still there. So I started the thread above, and was urged to report it to apple, as it was an unknown problem, despite my having reported it earlier. So I reported it to apple. And again with each iOS update that has come out since then - maybe 3-4 of them. It still hasn't been fixed.
There are several possible scenarios - first, that I'm the only one who cares about this stuff, and it's not worth the effort to make one person happy. That's fine, I'll live. Second, that there is indeed some kind of technical limitation that's preventing them from fixing this, and/or it is indeed some kind of drop of support for certain types of files - I haven't heard about that, so don't know. Third, that it's just too low on the list of needed fixes, and they'll get around to it later. Fourth, that perhaps my reports of this got lost in the shuffle, and they just don't know it needs fixing. Fifth, that there's just something wrong with my relatively new ipad pro running the latest iOS - although none of the "fixes" I've been offered have had any effect, and the files still work fine in all other media players except the TV app.
So I appreciate any feedback, but understand that this is something I can live with - the affected files run fine in VLC, I can watch them in that. I would prefer to watch everything in one app, but can deal with this minor inconvenience.