I have found multiple rendering issues with Illustrator 2020 on Mojave 10.14.6
There are three types:

1.) When cutting an object with the path finder, it might occasionally produce straight lines instead of curves, although I can't replicate this and it happens rarely.

2.) The displayed object doesn't follow the outline accurately in general. This is blatantly obvious when zooming in to the max and drawing a small object.

3.) Lines aren't smooth, they have extremely tiny irregularities. These disappear when zooming (even when already at max) but only for the current screen. Moving the camera reveals new ones and point 2. stays always.

From what I tested, saving to .ai or .svg carries over point 1. and recalculates 2. and 3. Apparently that's just an issue Illustrator has in general?
Saving to .pdf or .png seems to fix everything, making these bugs only annoying when an Illustrator file is required.