>>>>do you have Alfred or Unclutter?
No, I haven't.
>>>>what is different about acad's copy/paste that is different than other apps?
Obviously, there are some differences, but I do not know exactly what is under the hood. For example if you will copy something from AutoCAD and will try to paste to some other application on Mac, you will get only the path (in plain text) to the temporary dwg file containing the objects you copied from AutoCAD. Maybe that is because there is no OLE technology in macOS, and only AutoCAD can recognize real DWG data in the link, saved in clipboard.
Clipboard managers can potentially destroy some bits in the data copied to clipboard from AutoCAD and as the result AutoCAD cannot recognize it correctly.
Maxim