I just saw your reply, it's odd that I didn't get an email notification. ok, I just checked my settings, and email notifications were all off. I enabled them now. A minimal sample project is already attached to the bug report FB13810482, and it fully reproduces the issue. Do you not have access to it? If not, I'm happy to post it here.

It's hard to test originalPurchaseDate also, my TestFlight build returns 2013-08-01 and my app has only been available since 2023 May. And I agree with the OP, documentation is confusing. My TestFlight build originalAppversion returns 1.0 for both Mac and iOS. But for the local debug builds I get the current version number 4.8 for macOS and I get the current build number 1 for iOS. And the purchase date is 0001-01-01.

Correct, GarageBand on iOS (and even on macOS) doesn't yet support MIDI Processor type AUv3s. GarageBand for iOS is overdue for a major update, maybe we'll see something in the near future. Your best bet for testing is Logic Pro on macOS and AUM on iOS.