Leaf blower with outlet through plywood board on roof vent
Did you measure the difference in pressure between inside and outside?
My manometer showed a difference in pressure between inside and out of 1 inch of water, which is about 0.04 psi. This was with the dash fan and the leaf blower both on. Not much pressure, but it doesn't take much to get air flowing through a leak and creating a bubble in the soap solution on the outside. A higher pressure difference would have made more bubbles, but any bubble will show a leak point that might allow water to enter.