I did not yet investigate further and don't know when I'll have the next chance to do that, but if anyone beats me to it, I assume that bisecting 6.6 to 6.7 will reveal the root cause. Who knows, it might be a small thing. You can find my full test matrix (kernels, firmware) and notes here [2]. I have dmesgs from before and after tests (where applicable), but haven't analyzed them yet. I'll eventually host them somewhere. I just thought it might be best to share the intermediate results.
I confirmed the base case. v6.6.15 works and 6.7.12 is broken.
I started the git bisect. It's over 10,000 commit difference and will take about 14 steps.
I'm documenting the process here.
https://hackmd.io/UINQqeaTTOSRNv1yhFjVqw
-- Brian DeRocher