FWIW, I decided to test Larsenex's theory of "no precursor anomalies" by starting a new game, cheating up 30+ colony ships (with cheated up population) and then sending into the wild blue yonder.
Did absolutely nothing else except cheat up a couple of techs for speed and cargo space as well as turning off the fog of war. Didn't survey anything. Didn't choose any ideology traits. Just built some buildings on Earth and made all other planets either Economic Stimulus or Cultural Festivals.
On planet #32, stopped getting colonization events.
I did absolutely nothing this game except settle planets. I even made sure not to settle a single precursor world as well. So whatever happened in Larsenex's game, I couldn't replicate. Just that the no event bug is still happening in mine at the point it always does.