Ok, I first thought maybe it had something to do with a viewport bug, but when I tried many different viewport views, such as fullscreen, popped-out, and standard in-the-4-view the Canvas undo feature worked fine.

Here is my test pic, even though it probably wont help


By any chance are you painting your strokes, then making adjustments, then trying to perform the Undo after something has been changed?

OR, might you be painting more than one stroke, and then trying to undo more than once?

I ask because I had a student once who wondered why multiple-undos were not possible in the canvas view.

Darn Jo, I wish I could figure out whats going on.
Any chance of you passing along your file, so I can weed-out any chance that the file itself may be corrupt thus contributing to some sort of issue?

Thanks again!

Oh, have you tried doing the same thing on a totaly new file after having restarted Maya?

I'm just shooting in the dark here