Title says it all.
How to reproduce(using iron as example):
1. Setup a station with filter miniloaders unloading, filtering only iron.
2. Put anything else into a train that is NOT already at the station with miniloaders.
3. Send the train to the station

If you put anything else other than iron in the train while it's already "docked" at the station the miniloaders will not let it through, as expected.
I've tested it only with the fast filter miniloader but I suspect it's the same for all other tiers.

If you are using loader redux or a similar loader mod, you need to turn off cargo wagon integration or the unloading will be taken care of by Loader Redux, not Miniloaders.