We use the official kernel packages from RPi Ltd, so it is not on our control. However, I remember something: DietPi RPi4 HDMI audio output limited to 16 bits - #2 by MichaIng
Did you guys try to use the KMS HDMI device? We will likely switch to KMS by default (like RPi OS does) soon, so this is anyway what must work earlier or later.
Of course we could additionally open a PR at the RPi kernel repo and try to implement the change for the onboard driver to enable 24bit support. Will be interesting to know whether/why it was disabled (intentionally) in the first place: https://github.com/raspberrypi/linux/blob/rpi-6.1.y/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
