I followed your steps while running MorphOS from PATA and couldn't reproduce your result. I think you should file a bug report. This way, the MorphOS Team could determine with your help whether it's a hardware problem at your end or a MorphOS bug.
Do you use MOS 3.6 with Chrysalis pack 3.6.1 beta ?
Because during my hardware experiments and fresh installations I realize that with an blank/clean MOS 3.6 I had also NO issues with the screens. Just after installing Chrysalis pack that was of cause an still an beta I get this problems.
Anyway now it is MOS 3.7 out and also Chrysalis pack 3.7.1 beta and I will repeat my tests with this version in the next days. In Chrysalis pack 3.7.1 Papiosaur removed something in the Ambient prefs that "caused problems". Maybe this causes problems with the screens as well? We will see.[ Edited by Waitsnake 07.08.2014 - 13:46 ]