Nothing, all my colonies are at 100% but the timeline shows it decreasing, and the approval rating in the top left corner also says 95%.

Idk if it helps but it's a custom race, and the only other synthetic in the game, also custom, had the same problem. According to the timeline. 

I've seen that for a long time with non-synthetics. The approval in the top right never gets to 100% even though all of my colonies are at 100% and have been for most of the game. At first I thought maybe it was an average over the whole game but then you wouldn't see it going down once all your colonies were at 100%.