I don't know if it's intentional are not but for some reason whenever my city reaches a new level it's population seems to drop. For example when a level 3 city hits 500 population and levels up to a level 4 city I check it's population and it's now in the high 400s. So where did the several dozen people go to?

First time I noticed this happen my city also seem to upgrade my huts to houses on a low level city. So I thought maybe it was a loss due to the cap dropping then raising back up from housing change. Which might also be part of it but it seem to happen whenever it levels up regardless.