Nests

I have noticed the same thing in my area. Some of the places that were nests before are no longer acting like nests. Other places still seem to have a dominant/nest species but the relative spawn rate of the nest species seems to be less than before.

Around here, one nest group that spawned two species (so I'm assuming two adjacent/overlapping nests) has been changing dominant mon repeatedly since the end of the psychic event (during which, one was a Drowzee nest.) It now appears to have a Chimchar nest (based on spawn points and timing) in addition to three other species, for a total of 4, not 2.

One of the update features was listed by Niantic as less dependance on the weather and a wider distribution of pokemon spawns. So you'll see more stuff in general, less weather focused, and it should be harder to gauge if there's a nesting pokemon or not. No surprise, and a welcome change if your neighborhood is constantly nesting Sentret, Whismer, and Rattata.