I just purchased the game and ran into this issue immediately on a 2019 16" MacBook Pro.
Here’s a picture of what I usually see when I log in (I had to take a photo because if you take a screenshot the graphics briefly return to normal, as OP mentioned).
Thankfully, with a bit of googling, I was able to find a workaround that so far holds up for me. Find the executable file, right click it, select “get info” and then near the bottom there should be an option called “Open in Low Resolution.” I probably would have had to get a refund if this didn’t work!
That same googling revealed that a bunch of other games have run into this issue with Catalina. A few of those devs posted info about the fix in their patch notes. For example, a Jan 23rd update for Klei’s “Oxygen Not Included” included this in the patch notes after several players complained about the same thing:
" * MacOS builds now automatically choose between Metal and OpenGL graphics, this should prevent the fullscreen glitches on Catalina"
I’m not enough of a tech person to know that means anything in this context, but I hope that it can put Eleventh Hour on the scent for a fix!