System Restart Issue

At least you found the problem device and know how to fix the issue.

Can't you use a HDMI to HDMI cable? Is the Vega 64's HDMI display out port already in use?

With HDMI, when system wakes up from sleep, the display signal does not get restored. I have to unplug/replug the HDMI cable every time.

I use a DP to HDMI adapter plus the native HDMI connector on the iMac 4 system in my signature, via a Vega 56 to run a dual screen setup. I don’t have issues with wake from sleep. Nor do I have to unplug and reconnect either cable.

I use/own a pair of Benfei active DP to HDMI adapters, bought from Amazon.co.uk.

I am using a TV as monitor. Could that be the reason why? Is there anything in my EFI folder that would cause the HDMI issue?

The TV could be the issue, as it isn't designed to wake from Sleep from a PC/Hack, like a PC monitor is.

I doubt the issue is with your EFI contents, as you would more likely have a Black Screen issue when initially booting in to macOS.

The Vega 64 doesn't require any specific config.plist settings/boot args, like the Navi cards. Just a reasonably recent version of WhateverGreen.kext.

There are PowerPlay table options for fan speed & temp control, but nothing I recall for fixing a black screen after wake issue.