My project used VALID_ARCHS. Removing only that build setting in Xcode 12 made the app runnable on the simulator. Removing VALID_ARCHS required first clearing out its setting values. Then I was able to delete the entry itself and successfully build and run on the sim.

Last build to work for me was uploaded around 8:30 PM UTC. Then a build uploaded just before 9:00 PM UTC still hasn't shown up in App Store Connect -> Activity yet.