well im almost sure i have killed waaaaay more than 500 and i can swear they can appear almost anywhere, i won't dismiss what you said, but from my own experience there's almost no constant, i have killed the same falcon paladin thrice as fiendish and i have killed different falcon all over falcon bastion, of course i got some repeated, aswell i got some that i have never went before, an easy way to know this is demona, such a HUGE place can have repeated a wasp which will lead to a designed path, or could be a goblin 3 sqm far, behind a wall, with literally a totally different path.
so yeah i think u got rng rather than actually a pattern, but who knows.