Using Xcode 12 beta4 generated custom frameworks.
frameworks are successfully compiling in iPhone devices and same frameworks not compiling in iOS simulators.
In Xcode 11 custom frameworks are working fine for both devices and simulators.
Xcode12 beta4: Custom frameworks not compiling in iOS simulator
We had this same issue and solved it by tweaking the VALIDARCHS to only build x8664 for iOS sim debug builds. Obviously not a long term solution with arm64 Macs on the horizon.