Population not working correctly?

[LuciusSuperbus]

I'm confused with the population mechanic right now. I had plenty of farm houses all at 25 pop which is the max, and then all of the sudden it all dropped to 10 pop with the pop cap still being 25. They have full needs and luxury met. I'm curious why this is happening

Is it because there's pollution? Or because I upgraded some houses to workers which don't have full needs matched? I have 6 islands and none of them seem to have max population for the houses i have although the farmers show max with everything they need/want. (This is just a question because I'm trying to make my secondary islands into production hubs for basic goods so my main island can be mostly cultural and high tier houses)

[LuciusSuperbus]

Never-ending I just found out about the lifestyle needs based off of other population you have. It still doesn't explain why my other islands suffer when it's nothing but a farm community. So I should ask are lifestyle needs based off island pop or overall pop?

[XceptOne]

Without DLC (meaning without lifestyle need) using only basic needs (luxury needs don't increase pops, only money and happiness) you have following max pop caps.
Farmers 10
Workers 20
Artisans 30
Engineers 40
Investors 50
That's also the number you have when the building is ready to be upgraded (all basic needs fulfilled) as well as the number of pops that the next tier building starts with.

Lifestyle needs are additional and optional. They get usually unlocked by unlocking the respective wares. Other than basic needs that a building tier starts with, not fulfilling lifestyle needs won't have negative consequences. For farmers lifestyle needs are worth an additional 15 pops increasing the maximum to 25.

Strange part is, without actively activating and actuallly fulfilling all 9 lifestyle needs your farmer houses couldn't have been at the 25 cap.
That said, fulfilling all basic needs (=10 pops) is still enough to upgrade houses.

Just for reference, engineers and investors can get even more pops and needs by upgrading to skyscrapers.
It is also possible to go over the pop cap by using certain items in a town hall. When going over the cap the building will also start consuming more wares for each of its' needs, whereas consumption will stay constant while at or under cap.