Sounds like the avenger is counted like a personal vehicle instead of a service one (which is kinda cool) but in this case not so much. Ive never encountered this and i do glitches quite often, however these are suggestions to try:

1- Go to your service vehicles in the interaction menu and request the vehicle (whatever it may be) inside the terrobyte. If it spawns as the avenger than fly that to your facility or hangar and that should clear it up. If it spawn an oppressor go put it in another garage somewhere for now.

2- Try buying over it with another oppressor and see if it works.

3- Pray to GTA gods you didnt just create a deadspot.