I'm curious myself about why the filter is directional.
All of the cabin air filters I've used have had arrows (should point down). But it's just made of that paper filter material and seems symmetrical up or down, unlike the other kinds of directional filters (home furnace) where they are not symmetrical due to having a reinforcement on one side only. The Kia filters don't have any reinforcing, just the paper.
I had to call about 45K 2.0T New Spark Plugs -- and was also told Down for Cabin Filter Flow Arrow.
Dino, I reasoned the outside Air "Tunnel" Could go beneath the Filter and blow air Up thru the filter and into the 4 dash vents tunnel, so there wasn't any logic I felt comfortable with to Know. The bigger opening Below the Filter makes the Down understandable once you know the answer. - But I'm never confident 2nd guessing engineers.
You were right on Down arrow direction.
You can see how the dirt, leaves, and crap falls onto the top of the filter when you take out the old one, so it's kind of gross thinking you are sucking your "fresh" air through all that crap. But it also helps to think of it that way, the crap gets held back on the top, while the air flows down and through.