One oddball thought (I'm good at those
):
Is it possible that some deck leakage (say from a leaky scupper fitting) could run along the underside of the cockpit, or down the outside of a hose or wire, and drip right into the flame arrestor?
I know it's pretty unlikely, but with a clean pressure check of the manifold & block, it seems like the only other ways for water to get in are the intake or the exhaust!