Strange Grid-Like Artifacts Appear While Painting in Painter 2020

Hi All,

When I am painting brushstrokes, strange grid like artifacts will appear, making the shading cut off against a straight line. The even bigger issue is these artifacts won't go away after I hit Edit>Undo or close the program. I have to keep manually blending them back in the artwork. I'm not sure what I can do to fix this. I took a screenshot to show what I'm talking about. Does anyone have any advice? I really want to like this program, but I just keep having to deal with more and more issues and it's starting to feel not worth it.