We have seen this issue on some specific mac models starting from macOS 10.15.4. It is likely to be graphics driver issue with Metal engine, and we are researching what we can do to fix it, or we have to escalate to Apple for a fix in the graphics driver in next OS update.

You can use the workaround below, to switch to OpenGL engine:

Click menu AutoCAD 2021 > preferences > in General tab, change the graphics engine from Metal to OpenGL. Then restart AutoCAD. The issue should be gone.

Wei Ma
AutoCAD QA Manager