I once travelled with a commuter backpack (28L) and a duffel bag (we’d been travelling through other states beforehand). We didn’t have time to take the bags to a luggage storage place, so we went to the theatre (Music Box) with all of our bags. Security had an interesting time trying to go through our bags but they didn’t give us too much hassle (might’ve been because I was a student/looked young enough). I fit the duffel under my seat pretty easily (I think it was sticking out a bit), but I fit the backpack between my legs, so it wasn’t too bad – just a bit of a tight squeeze to make sure anything wasn’t intruding other people’s spaces.
The next time, I travelled with a rolling carry-on and brought it to Schwartz before the show. Best 10 dollars I ever spent, especially because it was raining really hard that day. I still brought the same backpack – didn’t have any issues except for needing to take a bunch of stuff out for security to look through.
So I think you should be fine even if the bag doesn’t completely fit under your seat – as long as it fits between your legs/feet, you shouldn’t have any issues.