Muting etc, all the same. Slider OK. My PC is tuned to the hilt, Windows 10 LTSB without all that MS c-o-c-k & Bull.
Sonnet USB card with four controllers, one for Audio only.
It has to do with RB IMO. Some setting maybe.
I'll look into it later. as i decided to continue the RB project once again in Cubase instead.

ADDED: Tested Realband 2019 v5 and 2020 v1 ...same thing.