Kitchen exhaust fan (built into microwave) pulling air from wrong place

I moved into a 10 year old townhome with a built-in microwave that has the range exhaust fan built into it. The fan is supposed to vent out the back, through the wall. There are two grease filters on the underside of the microwave. There’s a ton of grease buildup in the cavity above the grease filters (when you remove them), as well as in the upper part of the microwave where there’s a grille. If you remove the grille you can see the charcoal filter that is supposed to be used if the microwave is venting internally without an external exhaust, and it’s covered in grease.

After playing with things a bit more, I think the exhaust may not be installed properly, but I’m not sure. Some air does definitely vent out the exhaust pipe to the outside. However, the air is being sucked in through the top grille over the microwave rather than up through the grease filters beneath the microwave. I expected it to be sucked from below.

Can anyone confirm that the air should be getting sucked up from below rather than from above the microwave? If so, any ideas what could be going wrong?

Thanks!
Josh