Max boosts are:
100% for parks
100% for Education
100% for Transportation
100% for Entertainment/Gambling/Landmarks
I got this issue as well in the 2.5M city that I've built.
First, Education boost was 450k. Then I destroyed 2 universities and places residences. I didn't like the results and I placed the universities back. IN THE SAME SPOT. Education boost dropped to 413k.
Makes no sense. I could see some buildings getting half a bar, others no bar at all.
I know right?! And the population changes everytime after I go to someone elses city through trade HQ then back to my city, while no change was made to my layout whatsoever.
I've another bug that might be related with what you said which is when I go into the game again after I was already in, I lose pop.
If I kill the game's process to re-started, my pop is back to normal -_-
