As mentioned in this post https://forums.elementalgame.com/435073/get;3261574 I have seen the AI (on beginner difficulty no less) magically spawning 30 population cities with no expenditure of a pioneer.
Find 2 save games showing this issues at
q30 has a Magnar army of 2 champions plus a pioneer about 8 tiles south of New Pariden (you will need to reveal the map to see them).
q31 shows the same game a turn later. A city has been founded on the tile which still contains the army and pioneer. This city has 30 population.
I clicked through a number of turns after this with the map revealed and in that viewing I saw the army and pioneer proceed south and magically found another 30 population city without expenditure of the pioneer.
I also saw Yithril found a city normally and Magnar found one normally with a different pioneer up north of Imperium.
Hope you can track down this issue for us.