Q: Having problem with ouside air coming into the A/C

We are having a problem with our A/C in the car. We are thinking about taking it to the mechanic. The issue is that we're getting random whiffs of outside air, as if the recirculation isn't fully working. When we're driving, the air seems to be super cool and dry like normal. So it isn't a problem until we get behind a vehicle with lots of exhaust or anything with a strong smell.

Also, when we're stopped at a light or idling, the air gets slightly more humid than usual. We haven't replaced the cabin filter recently. Could that be it? Or something else?