Yeah, I'm pretty sure I have the same problem. For my "back" mouse button, I created a macro for the input ctrl-tab, and I'm trying to make it so that it waits until the button has been lifted, and also lifts the inputs ctrl and tab. I'm trying to do this via "insert new event" and setting it to "mouse event," but although "left up," "right up" and "middle up" are listed as options, there are no "forward up" and "back up" options, or even "forward down" or "back down".
Is there any way I could have the same functionality via some other option?