I solved this issue. The problem wasn't that my Other Swift Flags settings weren't being read. The problem was that I hadn't set the Build Configuration for my scheme's Archive action. It defaulted to "Release" when it should have been set to my custom value.

Here is the call stack: *** First throw call stack: ( 0 CoreFoundation 0x00000001804ae138 __exceptionPreprocess + 172 1 libobjc.A.dylib 0x0000000180087db4 objc_exception_throw + 56 2 UIKitCore 0x0000000184c11bb4 -[UINavigationController pushViewController:transition:forceImmediate:] + 3736 3 UIKitCore 0x0000000184c10bac -[UINavigationController pushViewController:animated:] + 756 4 SwiftUI 0x00000001cc954f7c OUTLINED_FUNCTION_4 + 109100 5 SwiftUI 0x00000001cc95c274 block_destroy_helper + 8052 6 SwiftUI 0x00000001cc949a54 OUTLINED_FUNCTION_4 + 62724 7 SwiftUI 0x00000001cc952ca0 OUTLINED_FUNCTION_4 + 100176 8 SwiftUI 0x00000001cc94e8f0 OUTLINED_FUNCTION_4 + 82848 9 SwiftUI 0x00000001cc1f7c2c OUTLINED_FUNCTION_14 + 28 10 SwiftUI 0x00000001cc7c0f58 OUTLINED_FUNCTION_16 + 15280 11 SwiftUI 0x00000001cc7c0ab0 OUTLINED_FUNCTION_16 + 14088 12 SwiftUI 0x00000001cc7bf25c OUTLINED_FUNCTION_16 + 7860 13 SwiftUI 0x00000001cc81c970 OUTLINED_FUNCTION_20 + 42792 14 SwiftUI 0x00000001cc81c9b8 OUTLINED_FUNCTION_20 + 42864 15 UIKitCore 0x000000018590a128 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1492 16 QuartzCore 0x000000018a1ae654 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 440 17 QuartzCore 0x000000018a1b940c _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 124 18 QuartzCore 0x000000018a0f2d30 _ZN2CA7Context18commit_transactionEPNS_11TransactionEdPd + 460 19 QuartzCore 0x000000018a122aa8 _ZN2CA11Transaction6commitEv + 652 20 QuartzCore 0x000000018a123f7c _ZN2CA11Transaction25flush_as_runloop_observerEb + 68 21 CoreFoundation 0x000000018040de34 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 32 22 CoreFoundation 0x0000000180408838 __CFRunLoopDoObservers + 528 23 CoreFoundation 0x0000000180408cf0 __CFRunLoopRun + 968 24 CoreFoundation 0x0000000180408514 CFRunLoopRunSpecific + 572 25 GraphicsServices 0x000000018ef06ae4 GSEventRunModal + 160 26 UIKitCore 0x00000001853e8040 -[UIApplication _run] + 868 27 UIKitCore 0x00000001853ebcc8 UIApplicationMain + 124 28 SwiftUI 0x00000001cc1fcb68 OUTLINED_FUNCTION_65 + 492 29 SwiftUI 0x00000001cc1fca10 OUTLINED_FUNCTION_65 + 148 30 SwiftUI 0x00000001cbebb068 OUTLINED_FUNCTION_0 + 92 31 <ourAppName> 0x00000001026d1f44 $s16<ourAppName>0aB3AppV5$mainyyFZ + 40 32 <ourAppName> 0x00000001026d2744 main + 12 33 dyld 0x00000001036e9544 start_sim + 20 34 ??? 0x000000010385a0e0 0x0 + 4354056416 35 ??? 0x2619000000000000 0x0 + 2745225447859027968 ) libc++abi: terminating due to uncaught exception of type NSException