It seems quite random when blender resets a position using Alt-G. Often Alt-G does nothing. When it works the viewport shows a reset ball but the graph editor shows a key being inserted at the same y-value as the previous key. If i then scrub the timeline the Viewport undoes the reset too, to fall in line with the graph editor. I get a key, but the position is the same as the previous key. Halp...
Interesting! As far as I can tell, that's a bug. Unless you have auto-keying off? Please report it here: https://developer.blender.org/