Omnigraffle screen pixelates/distorts badly while zooming

Hi. I have recently started using OmniGraffle. I have v. 5.1.1 Pro running
on OS X 19.5.6. I just started having this problem when I woke up my Mac
from screensaver mode and the Omni page view was all gibberish, with many
distorted sections. This only affects the canvas area, not any of the tool
palettes, and comes and goes as I zoom in and out. It seems to be mostly
happening at zoom levels greater than 100% (I am using Cmd+mousewheel to
zoom in and out). I tried restarting Omni, making a copy of the file, etc.
but it is still happening. Any pointers to this?

Tks.

F.