We've updated to Xcode 12 and the latest version of Visual Studio for Mac (v8.7.8b4) that supports it. Running unchanged source code in the simulator results in this behaviour:
Deploying the app to a physical phone results in no controls whatsoever reacting to taps. I've updated all nuget packages, cleaned, and rebuilt and the same behaviour persists. We have a spare Mac still on Xcode 11 and the same code (attached) works fine on there.