I just turned on my iMac now (it's been about twenty minutes) and the graphical issues came again after 40 or so seconds. The temps module in the Screenbar reports 47.4 degrees C, so it's not heat-related at all. Opening the Thermals program specifically reports the Radeon GPU's temperature as 41 degrees C and the CPU is idling at 2% usage.

Current assessment:
This bug appears to be reproducible by reboots. However long the iMac must be turned off before using MorphOS 3.20 again is unknown, but apparently it must be over 20 minutes, at least. But after that time lapses, I am able to use the iMac for as long as I am allowed to by the MorphOS 30 minute time limit, at least as far as I can determine. There could be something else as an unknown variable that contributes to this.

Overheating is not a factor in replicating this graphical glitch.

I am likely to try out NewSense's solution next.