MP 1,1-5,1 RX 590 Nitro+ showing strange artefacting during preboot

[flyproductions]

As the title says:
I'm running a Sapphire RX 590 Nitro+ Special Edition in one of my 5,1s, which i recently purchased. The card works perfectly fine in macOS as well as Windows. Even extended benchmarking without any issues.

But, while showing the GOP or OpenCore bootscreen, it shows some strange artefacting (see pictures). The green rectangle comes first and after that also the icons of the bootdrives appeare distorted. As soon as the OS comes in, no matter if Win or Mac, the picture turns crisp and clear. So i would tend to exclude any damages of the card itself. It happens, with any of the connectors and any display i tested (Cinema HD 30", Cinema HD 20", 27" LED Cinema Display).

But i have never seen anything like that with any other card before. Are ther any known solutions or anybody else who experienced similar with te same or other cards?

 

[flyproductions]

Finally somehow "solved":

It was just the ROM which was on the card at purchase, that seems to be kind of screwed. Even having the same revision number and release date as the most recent one TechPowerUp's VGA BIOS Collection has available for download, it shows some minor but, as seems to be the case, important differences.

So i injected the "Enable GOP"-image to the one from TechPowerUp, flashed the result to the card and now everything works pretty fine.