In the end I traced this crash to something related to the Secure Enclave ECC encryption that I’m working with, but then it was inadvertently resolved by another fix that I implemented, and that other fix may well have had an effect on the interweaving of background and @MainActor methods. I don’t know to what extent Secure Enclave encryption may involve the Objective-C-related things that expose one to these @MainActor foot-guns, but I just wanted to post this here in case the information I’ve provided sparks any helpful feedback in response, because inadvertently fixing a completely cryptic crash that reproduces 100% of the time on my boss’ TestFlight instance and 0% of the time on my devices is unsettling…