FWIW this is not actually an issue with default main actor isolation. I previously mentioned in the amendment thread that we found that most of the issues we were dealing with were fixed by reducing how much we baked sendability into the library's protocols.
The issue above is just that with some recent changes we accidentally broke default main actor isolation. The fix is simple enough, and really the biggest problem for us has been having to support multiple modes of Swift execution while building Swift libraries. This is exacerbated by the fact that as far as we can tell, SPM does not actually respect the .defaultIsolation(MainActor.self) setting and so it’s also hard to get automated testing for that mode.