I threw away an old LG LA-Q379SB filter, didn't try to wash it. Should have tried to wash it and see what happens.
But seriously, by right washing shouldn't be able to get most of the PM out. What the original LG filter is that the construction of the media has a tougher outer surface that can withstand abuse. If you have touched borosilicate filter media before (the stuff that most cleanroom filters use), it is extreme delicate. Your fingernails can create a puncture on it just by slidng along the pleats.
Washing and physical cleaning actions will compromise the HEPA integrity.