Good news!? macOS 12.0.1 Monterey based build shows good decimal format results. Xcode is still Version 13.0, 13A233. Its' funny experiences but I have to say OK. It's only checked on storyboard, I'm not sure SwiftUI.

This issue also found storyboard textField, numberFormatter, and Decimal Style selection. I open lagacy project on Xcode13, build, then comma separator does not work.