In my actual game I decided to change my season length for all 3 seasons to long, but Humid Season just started on day 12 - the same day as it started when I played with default settings.
I'm not using mods atm.
Edit: After further testing I just realized that I can not change anything in the world generation options. So this bug report is a duplicate to this:
I'm very sorry; I guess I searched for the wrong key words. This bug does also apply to Not-Hamlet worlds as long as the hamlet dlc is enabled and loaded.
- start a new game - adjust anything in the world generation options - let world generate