There are long periods where the filters aren't running during startup and shutdown, you'll notice once the seatbelt sign turns on at the beginning and right af

There are long periods where the filters aren't running during startup and shutdown, you'll notice once the seatbelt sign turns on at the beginning and right after it turns off at the end, things get stale super quickly and the temps start rising. That's because they turned off/haven't switched to air supplied by the jetway yet.