Make a copy of your england campaign directory (in case you make a mistake)
Then open the editor and load the Falkirk map.
Select a schiltron (the 400 pikemen unit) and press F11. You'll be able to edit the number of people in the unit.
Reduce the number to 2 and you will not have the bug.
I will try again this battle after the release of the patch but looks like a really easy one
