Hello Cactus.
I see the problem. It seems that items in png files are not loaded or refuse to be displayed. This is the first time I've heard of this problem. Unfortunately, I would not be able to make the link between this bug and MacOS 10.15.7. Sorry Cactus, this game is almost 4 years old now, it was my very first, and it remains imperfect in certain very specific cases that I have never been able to determine. I don't know how to help you.