There seems to be a lot of this, just in different forums.
Mouse Click Issues - Support / Bug Report - World of Warcraft Forums (blizzard.com).
I’ve got autorun on button 4, walk backwards on 5, and autorun seems to default to walk backwards if my last manual movement was to walk backwards. Nor does autorun cancel on manual input (trying to run forward by pressing down left and right at the same time). The situation seems to resolve itself if I swap the bindings on 4 and 5, though its spotty.
Consequently, I’m easy pickings for ground effects 
Bindings have been the same since Wrath.
I’m not a programmer, but I expect there’s some variables in the autorun direction and state that are being modified by directional movement and prior usage, rather than reverting to “run forward until canceled by toggle or manual input”.
Edit: whenever it’s wonky on the mouse, it also gets wonky with my keyboard movements. Clicking my keyboard autorun causes me to run backward if my last movement was running backward.