Same problem with MacBook Pro 13" 2017, and I tried these actions:
Wipe & install macOS Catalina, the problem was gone
Upgrade to Big Sur, the problem came back again
I guess the problem was caused by a software issue, since the brightness function is ok in Catalina.
Zzzzhs
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution