It’s not completely ‘blobby’, so we have some hope there.
Most control of HDMI is done via /sys in userspace, so it’s possible for us to strace kodi.bin and see what sequence and commands are being sent to the kernel.
Can you take a photo (crude I know) of video and audio settings? Then I will see the exact sysfs interaction between Kodi and playback for your settings and can probably narrow it down a bit.
Sam