I swear allegiance to the only one true storage vendor, Micropolis. The Micropolis 1323A being the embodiment of perfection in storage basked in the glow of the only holy storage interconnect, MFM.

I wait patiently for the return of Micropolis so that I may serve as their humble servant.

Your bridge isn’t bridging properly. If Router B is sending a destination unreachable then the packets are being handled on it further up the stack at layer 3 by some sort of routing component rather than by a layer 2 bridging one.

Often on Linux group membership changes only take effect on login. So you could try logging out of your session and logging back in after your group changes to test that theory out.