there are 9 pins in a 3 by 3 layout
the middle row of pins connects either to the top row or the bottom row when you press the button.
View attachment 12575
so 2 connects either to 1 or 3
5 connects to 4 or 6
8 connects to 7 or 9
nothing in each column should connect to any other pins on the footswitch if there are no wires connected to the pins. it is 3 separate sets of switches.
one problem you have is that you already have a small circuit board soldered to your switch, so you need to figure out whether that circuit board also has connections between any of those pins even when no wires are attached. I have not looked at the layout for your pedal to see if that is the case, but the wiring diagram in your build docs only shows pins 1-2-3 and 7-8-9 being used, with no wires connecting any of those pins together. if your meter shows there are any other connections between the pins then the switch is bad or there is a problem in the small circuit board. 1-2 should be connected at the same time that 7-8 are connected, and 2-3 and 8-9 would be connected at the same time after you press the footswitch. there should be NO connection showing between 1-3 or 7-9.