Can you show the pop summary for those population - any issues with your pop production modifiers? Summary middle bottom.
Oops, sorry, that was it.
So ~10 million is when they start to feel unsafe and want to be protected by the military.
I missed this amongst all the "Unrest is rising due to overcrowding" events. I only looked at the event type, not the text and ignored those because more infrastructure was already on the way.
Once political instability hit the lowest level, there is no event warning about decreasing stability anymore because political instability hit rock bottom.
Maybe a "Unrest is as bad as it could be, do something!" event would be helpful?