Bosses leaving the arena

I've found that two bosses, at the very least, can just up and leave the arena.

* Viper Napuatzi can leave the circle of soldiers, in particular while they are advancing to narrow the arena. This can also happen to the player character.

* The third Sekhama trial boss (the scorpion) can dive past the exit door during its (way too extended) periods of invulnerability. It just leaves and doesn't return.

That's annoying but not broken for a campaign boss. It is extremely costly in both time and farmed resources in the case of a trials boss.