Okay, I just checked and in the saves posted in the thread I linked above Magnar has 125 gildar the turn before the city is founded (and only one pioneer in the army), and 126 gildar on the turn after and a pioneer present in the city.  So if the pioneer is rushed (which I severely doubt) it is done so for free.  Check it for yourself from the saves.  Note that they are 1.00.  If there is a free instant rushing bug at work that still leaves the founding population of 30 unexplained.  I think that there is only one bug at work in this case and the game is spawning a city without consuming a unit as if it was the sovereign founding the capital.  This explains both the persistence of the pioneer without any free rushing scenario and the 30 starting population.

Frogboy, given this I do not have much confidence that you have squashed this bug at all.   That there are cases of immediately rushing pioneers does not mean that all cases are explained by this behaviour.  See my saves for a case in point.  Without knowing more about the OP's game neither you nor I can tell if the AI is quite legitimately immediately rushing a pioneer or if there is a bug at work here.

signed,

Frogboy's harsh Logic Taskmaster