It looks like XCode 12 is stricter in checking this. Apple stated:
This framework isn't built with a supported configuration -- iOS and iOS Simulator code has never been supported in the same binary. The linker in Xcode 11 began identifying these incorrect configurations and issuing warnings, and Xcode 12 goes further in identifying these issues.
We're having issues when linking the SDK.
This looks a lot like Xcode 12.3: Building for iOS Simulator, but the linked and embedded framework was built for iOS + iOS Simulator
It looks like XCode 12 is stricter in checking this. Apple stated:
https://developer.apple.com/forums/thread/669411?answerId=652663022#652663022
Do you need to release a new Xcode 12 compatible framework?