SpaceX

Easiest answer is that they are using preburner exhaust to pressurize the tanks

Has there been any evidence that they are doing this? I've seen this mentioned in a number of places, but it seems mostly like speculation.

That said, I can't think of any other source of filter clogs, unless one of the workers left their hat or something inside. Perhaps Tim Dodd can clarify this during his upcoming Starbase tour.