Well, have same problem, OPI 5 always starts with HDMI disabled.
I'm enabling it with:
echo on > /sys/class/drm/card0-HDMI/status
just chceck the path, if it's same for you (writing from memory, might be wrong)...but first part is solid (echo on > /sys/class/drm/)
And I think it's from the moment, I have replaced monitor from pure HDMI to DP/HDMI one...so there might be problem with detection of monitor by OPI5.
But if anyone has a permanent solution, please post