Today Apple appears to have resolved the issue server side - I was able to upload my archive without recompiling from yesterday and it got accepted.

I have filed a request for this here: FB11810918

Anyone found any workarounds other than scheme duplication?

Xcode 12.5.1 seems to have fixed the issue for me: the first build and run took 30+seconds to attach, however the second and third build and run attached after around 3 seconds.

There’s a thread about this on Twitter too - but looks like I can’t share links to Twitter here. Someone has created a radar too: FB9091473