This occurred on a solo match of the starting room challenge mode. After an explosion from what I believe to be shatter blast via elemental pop, Tessie rose into the air and came down some distance away. A few rounds later I noticed I received an "enter Tessie" prompt and was able to drive her around the map killing zombies.
I could not break down the gates to Blackwater lake or Vandorn farm. Tessie sustained no damage from any zombie including Zursas. I would take damage if the zombies piled up too much, which I believe is normal. I could switch seats etc, and most importantly, exit and reenter Tessie without problems. It was hard to deal with Zursas from inside due to poor shooting angles as well as bees entering Tessie if I knocked off a hive. After a save and quit, Tessie was back to in her normal spot and I could not enter. Looking forward to trying to reproduce this.
p.s. I think I may have heard the Tessie garage repair sound shortly after the explosion.