A bug was reported with Studio 4.15 here https://www.avrfreaks.net/index.p...
I verified the it was not working with Studio 4.15 and as I use this feature frequently I posted an official bug report but did not even received an acknowledgment.
Hoping for a fix in Studio 4.16 I just installed it and now it is not even possible to click on the register name to change it.!! Novel way to fix things, if your head hurts we'll cut it off.
Watch does not work with the register for some reason, the stupid not in scope thing.
So I'll go back to Studio 4.14 for now and anyone else that needs that feature will need to do the same.