Comment # 11 on bug 996614 from Max Staudt

Hmm, seems like both the kernel and the firmware have been updated... do you
still have an old kernel and/or firmware laying around that you can try?

As for the firmware, you can also get previous "start.elf" blobs from here:
https://github.com/raspberrypi/firmware/tree/master/boot

AFAIK the RPi 1 does not use the 'vc4' KMS driver, so video should be under
firmware control. I cannot check this right now as I don't have the hardware on
hand.

I am afraid I won't be able to look deeper into this issue too soon. In the
meantime, maybe a firmware/kernel update fixes the issue.