Seem to have found an annoying fix.
With Xcode 12.01 (the latest stable release), I couldn't launch to a 6th gen iPad running iOS 14.1.
Nor could I install a simulator with iOS 14.1.
So.
I downloaded the 12.1 GM seed.
Still couldn't launch run code on the iPad running 14.1.
BUT, from the devices and simulators window I was able to add a simulator with iOS 14.1.
after which: I could launch!
I quit the GM seed, relaunched the stable release of Xcode, and it runs fine.
Annoying. But worky.