Is there any rhyme or reason to it or is it just RNG?

RNG I don't know of anything to manipulate it.

I feel there is a way, but i dont know how. A good friend and runner @Proffesorretro had a great to save state before the bonus and then go into the next stage. The coin consistenly spawned erd door from right for me. So its locked into a certain door based on actions you take prior i feel, not randomly assigned. We just need to find out if its based on time or total movements. All i know is for me it has around a 1% spawn rate, and in 825 runs to get my pb i got it first door about 9 or 10 times.. its soul destroying..