same as problem here with ATI 9800 flashed in PowerMac G5 and MorphOS 3.12 :( (if I remember correctly there was not this problem only in MorphOS 3.9 or 3.8)

boot with ISO, change resolution to 16bit in it, and find the first distro version that has this bug, probably the problem is from 3.9 or 3.10

[ Edited by Divinity 05.01.2020 - 22:09 ]