I have played around a bit with different BIOS Versions. The first version which gives me garbled Graphics is the version which has following in the changelog:
"Update AGESA 126.96.36.199 ABBA
PCIe Gen4 disabled when using a 3rd Gen Ryzen (Matisse) CPU due to this new AGESA"
All BIOS versions after that are bad for Batocera too with my Gigabyte B350 board.