You can also try toggling parts of the new generics infrastructure, as there may still be bugs on one side or the other. There are three different flags at different levels, and one part of the generics system was moved to the new Requirements Machine in Swift 5.6, so toggling the various parts off and on may change some things. I would try turning more bits on first, then turning stuff off. The "Requirement Machine", a new generics implementation based on term rewriting
Edit: Now that I notice the arch issue I wonder if there's an underlying LLVM issue since it seems arch specific. You might want to try a 5.7 toolchain and see if the issue still exists.