I've noticed this more than once as well.  I suppose it could be rushing a new one immediately.

I have seen an "army" including the facion leader, 2 normal army units and 1 settler. They built a city and the same "army" continued onward including the settler.

Fascinating ... I never suspected this to be the case

they already had another settler walking towards the spot . If that is the case and they rushed a new settler...I guess they have a VERY high inncome. This was very early in the game too. I wasn't even earning enough to just rush stuff so how can the AI do it. Seems very convinient

One thing I hated about War of Magic was the army of settlers, one after the other, after the other..., the never ending settlers.

I'm surprised to see that a settler issue persists with FE.

Also...how could they rush a settler at start if they first have to research technology which allows rushing?

No matter what the player does, they DO have to wait a few dozen turns before they can rush anything...

You can get the civics tech as a faction bonus for free, though.

perhaps th edev's could run a few test games where settlements have to wait 2 turns before they can start producing anything. It is possible the immmortal pioneer unit is actually an immediately rushed one. But i believe people have reported this behaviour with outposts as well.

Which is why when I'm at war with an AI and they send pioneers at me, I kill them the turn that I spot them.  Gilden likes using that strategy so he can figure out where to send his overarmoured minions.

I've reported a similar bug to the OP a ways back.

https://forums.elementalgame.com/435274

It would be interesting to me to know if the cities being founded without the expenditure of a pioneer were at 30 population instead of 3?

With regards to rushing I think it was very improbable in my case (using cheats you could check the saves I posted to see if the faction expended an appropriate amount of gildar but I'm pretty sure that it didn't...).  Turn 30 on beginner, I think at epic pacing.  Given that Magnar recruited a champion I'm not sure where he got the cash from.  Further that pioneer went on to found several cities before it disengaged from the army and got gobbled by a monster.  Maybe there was enough money for one rush but certainly not for another 6-8 turns later, and another after another 6-8 turns.

As far as I can work out the trigger is having a pioneer grouped with other units in an army, although it is possible that one of these units needs to be a champion. 

Frogboy didn't understand the problem.  In another post he claimed to be skeptical since he could not reproduce this.  Maybe he could have loaded the saves.  Should rename him Ostrich. 

It may be a little too early to tell, but I modded in some population consumption for building shit, and that has pretty much stemmed the tide.

Just watched the Kraxis do this in a game. It's no wonder the AI manages such stupid expansion rates.

I don't know if building an outpost will actually use up an AI Pioneer, it's worth knowing.

We looked into this. The pioneer is being consumed correctly. Either the AI is rushing or there were multiple pioneers in the stack.