Being able to view your MorphOS desktop may simply mean that the device passes through the display signal as-is to the HDMI out port / your TV. It does not necessarily mean that the signal is being successfully processed.

One thing that stood out in the specifications on the linked Amazon page is the following:
Input resolution: (...) 1920x1080(60p)
Capture resolution: (...) 1920x1080(30p)

So, while your device accepts Full HD at 60 Hz as an input, it only supports 1920 x 1080 at 30 Hz for recording.

In any case, my recommendation would be to switch resolutions and see if this changes anything. 1280x720 would be a good candidate, for instance.