Got this on another one, every other new colony actually. I wonder if it has something to do with the fact that they require Atmospheric Cleansing to colonize.

Replied with a savegame to the automatic email.

Quoting BuckGodot,

reply 2

This has been going on for a while now, unfortunately: lol all worlds get precursor colonization events

I sent in a ticket on this pre-1.6 and it's still happening to me.  I've seen it happen even before I get Atmospheric Clearing, so it might be a weird bug that is triggered after a number of planets is colonized.  Or a certain type is colonized.  Haven't found a set trigger yet, which annoys me slightly. I like my bugs to be repeatable.

We are still looking into the root cause of this.

This would be my uneducated guess based on 6 or 7 games of this happening, but happening differently each time. It seems to occur after 30-35 colonization's, usually around or before turn 100. The precursor events will be intermittent, and there will be several planets that get no event, and then it goes back through the list again. I do not know if the random events play into it somehow, either vanilla or mega events. I do play all of my games on Gigantic maps or larger, maybe that might even play into it.

Now that you mention it, I think I've been only getting one per type as well.  In my latest game I've now cycled completely through the precourser list and started getting regular colonization events as well.

I suspect this is in fact what is happening.  Why the game isn't doing them randomly and is doing them one-per instead, I haven't got a clue.

Checking ColonizeEventDefs.xml, I see there are 30 normal colonization events.  Take out no events for having your initial homeworld and any of the starting worlds in your home system and then add the occasional precourser planet event that does happen when it is supposed to, then I can in fact see why this is happening in the low to mid 30s for us.  Looks to be strong correlation at any rate.

Yes that seems to be the problem.  I've updated our case and will have someone look into fixing it.  Thanks!