I did a surface trim of Nauvis after discovering the entire planet. All newly generated territory spawns with regular biter nests are not converting to the rampant version. I've been sitting within sensor range of one for like 10 minutes, and it is still a vanilla nest.

Yes, I know about this bug.
I don't know exactly what is causing it yet. Can't say for now when this will be fixed.

With a new settlements, new nests will be converted, so that gradually the difficulty will return to the original

In version 1.7.3, after chunk trimming, nests will mutate.
Unfortunately, what was cut off before the release will not change. Re-trimming should help