There was a cluster of many Drengin worlds whose population were unhappy and many worlds were in a state of partial rebellion.
When I invaded the worlds I noticed that these still had the unhappy face emoticon around the planet even though I had 100% approval rate. The status still said that many of the formerly Drengin worlds were in a state of rebellion with various percentages.
At first I though it was something specific to the Drengin but it was not the case as after many moves these worlds have not rebelled. (I created culture starbases around those worlds just in case but no change in their rebellious status.