What session type/mode did it happen in, i.e. practice, race, LB Challenge? Any specific steps to reproduce this maybe? I gave it a quick try in practice mode.
I had this in a training session (not race weekend). I tested my messed up USB controllers so did only a short test until my USB ports failed again :-D maybe it changed the value but I had no indication in the cockpit, the displayed value stayed the same in the cayman dash (i drive in VR without any HUD element)
I'm testing in a single player training session, it doesn't change value at all (the display shows percentage but it doesn't move), if I come back to garage I see same value as before. But I think I noticed that in some other session types (maybe leaderboard). Button works fine with other cars.
Ok, reproduced the issue in the public version. I was trying in the tester version before, so the issue has possibly already been fixed internally. Anyway, thanks for your report.
Well well, here's an interesting update for you guys (and me, obviously I had missed that bit before, sorry). Turns out the public version of RR is correct. In the testing build I was using we still have an older iteration of the car, in which brake bias adjustment while on track was still working. Due to real life driver feedback that was changed however. Actually on-the-fly bias adjustment is an optional feature of the MR version of the car, however, the Trophy version of the car is just the Clubsport, no MR. ^^ Long story short, it's true to life. Cayman Trophy drivers can't adjust the brake bias while driving, but only while the car is in the garage.