Thanks @laibsch and @royk.
For now, we’ve managed to get things working by running the latest vendor kernel, but using panel-simple.c from version 6.1.43.
This workaround isn’t ideal, and we’ll try to investigate the root cause once we have more time.
As for the log message "Expected bpc in {6,8} but got: 0", it doesn’t appear when using 6.1.43, even though the relevant source code is identical in that part.