There must be something different in the way you and I have MainStage set up. By default the Concerts I use in MainStage pass midi through to all VI's, including UVI Workstation > B-5.
As noted above B-5 was working as intended with faders and rotary encoders, the only issue was with each of the buttons. And the buttons mapped to the other VI's in that Concert, also noted above, which wouldn't be the case if if midi pass through was blocked by MainStage.
The only other difference in our respective setups could be in UVI Workstation which has also been updated to 2.6.11, whereas I am on 2.6.10.
It would be good if UVI and AS could send an email to registered users letting users know a new version is available rather than relying on users to check this themselves from time to time.
MainStage 3 | Axiom 61 2nd Gen | Pianoteq | B5 | XK3c | EV ZLX 12P Hoping everyone stays safe and well and sees this through to the other side