Apple LED Cinema Display not working at boot

I have a super reliable OC 0.6.7 build running on a Gigabyte Z490M iGPU for the last 6 month. I was using a 2K Lenovo DP display. I recently connected a 27" Apple LED Cinema Display with a DP->Mini DP female adapter and its works great except that the boot screen, bios and OC boot picker are not displayed. It turns on fine when the OS loads and it sleeps and wakes perfectly.

Occasionally, I've had a problem where it won't shutdown properly and when I reboot, the display doesn't work. If I connect the Lenovo DP display, it boots just fine and then I can switch back to the LED Cinema Display no problem.

Can anyone shed some light on this? Is it a motherboard issue?

Thanks