For point on contrast similar thing happened with my chevy 1500 and it has never had a k&n. Lots of dust and particulates seemed to have gone past the filter amd were in the it tubeI had K&N air filters in all our vehicles that I could purchase one for - until 2 years ago when I ran my finger on the inside of the air duct between the MAF sensor and the throttle body on our 1999 Buick Century and our 1999 Silverado. The tip of my finger came up dirty and gritty. I went back to paper filters on everything!
Also have to question if the muck you had in your inlet tube was just oil that got sucked through and had caught particulates.
I only have run reusable filters on 2 cars. One Ford filter and 1 k&n. Both mustangs. Neither car had your issue.
I will say, one filter (ford filter) was way over oiled after the dealer did some maintenance and the car ran like garbage until it was addressed.