I don't know if this was supposed to be normal or what, but Tab behaves in a way that's not expected. One would think that it enters and leaves 'Canvas Mode', but it does more than that:

1 - Pressing it once enters 'Canvas Mode'.
2 - Pressing it again leaves the mode, but with Krita in 'Full Screen Mode'.
3 - Pressing it again returns to 'Canvas Mode'.
4 - Pressing it again unmaximizes the window and restores it to a small size.

This happens if the key is pressed from an Express Key or the keyboard itself and it's pretty consistent. It always does the four 'steps' in that order, then repeats every time you press tab. I don't think a single shortcut should change multiple states at once, as 'Canvas Mode' is independent from whatever state you left Krita before entering it.

Yes, that has been reported to bugzilla.