Answering my own question: this other thread https://developer.apple.com/forums/thread/702028 has the same symptoms (sudden crash on iOS 14, but only when delivered via TestFlight or AppStore, not when running through Xcode).
Interestingly, the solution here is "disable bitcode", the solution there can be "enable bitcode" (or switch back to Xcode 13.1).
N. Holzschuch
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