I've just purchased an AMD Radeon 6700 XT (reference model) and am using an older ASUS Maximum X Hero motherboard (Z370) with an i7-8700k.
This motherboard has a built-in BIOS POST code LED and when rebooting or powering on the system, it will randomly not start with the BIOS CODE "AF" being displayed. The motherboard manual describes this is "Exit Boot Services event" which doesn't mean anything to me.
For testing, I've setup a Linux environment which automatically reboots every 2 minutes. It might take a few minutes or hours but eventually the system will fail to reboot with that code.
I've replaced the card with my original Nvidia card and had no failed reboots even after 8 hours (~240 reboots).
I'm wondering if the AMD card is bad or needs a VBIOS update or has some incompatibility?
Are you using UEFI Mode in BIOS?
If you are, temporarily disable Secure Boot and see if the PC boots with the AMD GPU card installed.
If it doesn't then enable Secure Boot again.
NOTE: To check to see if your AMD GPU card is defective install it in another PC and see if it boots up or not. If it doesn't that would seem to indicate a defective GPU card but if it does then it is an issue with your PC.