The rulebooks explicitly say:
"Place new facilities in any eligible territory that you have controlled since the start of your turn. "
So you are of course correct, you are not allowed to place a new IC in a just converted friendly neutral territory.
This is one of the aspects that TripleA handles incorrectly when dealing with “neutrals”.
So the correct handling of the rule must be player enforced.
I move this topic to the TripleA Support category.