Playing WOTLK Classic, and I use a Logitech g600. I dedicated two action bars for my side buttons, with one of the action bars working when I hold ALT. While the action bar was still switching, my mouse buttons (num pad buttons) were not working for it.

The fix I found was to add another keybinding, holding ALT with the proper mouse button, and then saving. THEN, I removed all bindings, and simply rebound the numpad buttons as it was originally. Now, it works like before. Strange.

This should be resolved now.