In a mission, I entered into a Stationary Cannon via "E".

I started shooting at a Dreadnaught

The stationary cannon became surround by the enemy.

I attempted to leave it via "E"

It seemed the game tried to allow me to leave, but could not exit (possibly due to all the enemy surrounding it).

In either case, I died; respawned.

Upon respawning, I went back to the room and killed all the enemy; however, the entire Stationary cannon had disappeared.

Again, the entire platform was visibly gone; however, I could still bump into it (as if the model still existed, but it was invisible).