Is the boss spawn completely random?
Solution 1:
When Tripwire announced the patriarch and put him in Killing Floor 2, they announced that each map would have a 50/50 chance for either the patriarch or Volter. Tripwire wanted the final boss to be a toss up each round, so players do not know and strategize for a specific boss. This uncertainty adds a bit of difficulty to Killing Floor 2, which already is no walk in the park.
For reference, pcgamer published an article for the patriarch announcement.