Steve,
I actually have the MX Master 3 and have seen similar issues. I have gotten general commands to work with the side buttons, such as copy and paste. I haven't gotten to the point of reaching out to Logitech support quite yet, but would assume this would be the next best place to go. I have a feeling that it is less a Land F/X issue, and more a AutoCAD platform issue. There could be a chance that while AutoCAD comes up in the button customization list, the mouse doesn't actually know what to do once in CAD.
Would love to hear more from others if they are experiencing something along the same lines.
Cheers,