This is based on my personal observations of spawning S ranks. For the non-programmer, assume the word "flag" means "checkbox".

Every A and S rank has this flag that is basically labeled "is spawnable?". This is set arbitrarily by the server unannounced to the player at a random time between what we call the minimum and maximum windows. Until this is set, all work on spawning something is ignored.

In order for a mark to pop, all flags must be set. The Rank As only have 1 flag, so they instantly pop upon the "is spawnable?" flag being set.

For Okina, we have at least two other flags. They are "is full moon?" and (lets assume here) "enough yumemi's killed?". Some servers got the "enough yumemi's killed?" flag set before full moon, others did not. Those that did saw what appeared to be an auto-spawn scenario right at 12pm. The rest came here looking for answers as to what's up...myself included.