I have the ender 3 pro upgraded with the enderextender xl for a 400x400x500 print space. I made a firmware with the build plate sized appropriately and bltouch set to a 9x9 point bed leveling but when I go to try and level the bed the bottom row is in front of the build plate and not reaching all the way back and the left row is to the left of the build plate and not reaching all the way right so it is trying to probe nothing.
Not 100% sure - but it sounds like you haven’t set a probe offset?