It might be fixed… After no response here we just spent some happy moments going through our app stripping out everything we could to get the minimum reproducible case DTS require for a submission... And then suddenly it worked! And in the last stripping step we'd stripped out some non-code files… Returning to our full app that worked as well. Looks like we can now verify. Time will tell if this “feature” is really squashed…

It appears this bug has been fixed in Sonoma Beta 5 (which arrived while we were submitting a report in the Feedback Assistant…). Thanks Apple/@eskimo 👍