I'll play devils advocate and say that, sometimes, I've seen AL groups morph out of home games which need more players. Sometimes the DM's old friends will show up, and it's kinduve their choice to DM to begin with.

Also, if someone messes up and gives a walk-in a seat, it would feel really shit to turn around and exile them.

Now, that said, I'd contact the event coordinator and ask them to make sure the system is fixed in some way. If you have a significant drive or other time commitment, mention that.

The only other advice I can offer is to be comfortable with DMing every once in a while, which has saved me a couple times when that has come up; I'll just plop down, set up my own AL table at the event, and run people through an AL oneshot. Not great advice per say, but that's how I deal with it.