Thanks again, Eskimo, for your quick help. I did as you recommended and found the code signing identifier and bundle ID matched as expected for the app. In the /watchkitsupport2 folder there is one file named "WK" - I think that's the nested watchOS file you mention. When I run codesign for WK file:
Executable=/Users/davenorfleet/Downloads/MyFitnessDJ 2020-10-20 08-19-11/MyFitnessDJ/WatchKitSupport2/WK
Identifier=com.apple.WK
Format=Mach-O universal (armv7k arm6432)
CodeDirectory v=20400 size=453 flags=0x0(none) hashes=9+2 location=embedded
Signature size=4628
Info.plist=not bound
TeamIdentifier=not set
Sealed Resources=none
Internal requirements count=1 size=60
So it seems the Identifier, com.apple.WK, is being set here from something.
Also, although I use automatic signing for iOS app & watchOS app & watchOS extension, I get a prompt while distributing in Organizer saying that "appname_ needs to be re-signed for App Store Connect distribution." I select automatically manage to continue.
Does that give you any more clues as to what the problem may be?