There is just 0.05mm up to 0.1mm space (depending brand) between the crankshaft and crank case. Probably you will not imagine it but the crankshaft does flex. With an unbalance by default with a 1 cilinder engine and maybe from the clutch the crankshaft can band that much to touch the crank case. But just keep in mind with foam filters it is possible that dirt will come through.
I know in the large scale with K&N air filters they have a dust sack to put over the filter, maybe something to try.