@Tlaloc well, first of all, a huge thanks for actually responding at all. I can't get anything out of Apple.
I don't know what's going on with StoreKit 2, but testing it is not possible in development, it's so flakey (certainly on the mac, anyway).
Here's a post I did on Transactions on the Mac:
https://developer.apple.com/forums/thread/758315
It just doesn't work reliably.
I'm left thinking I'm going to have to use my live userbase for tests - i.e. try something in code and release it, then see if it works. I've been a developer for over 40 years and I've worked with a huge range of technologies, but I've never been in this position before.
I've been suggesting users download the app again from the store. I have a swathe of suggestions I've been offering, but nothing seems to work.
I've even bought our own app, and am running 15.4, but so far it works for me.
We also offer the app via our website, so I've been handing out free licenses to that version.
Your problem sounds bad, but at least there are things to try, even if they aren't ideal. I currently have no way forwards short of Apple fixing something, and that's not a position I want to be in.