I don't have a savegame for this one, I'll try to replicate it. What happens is that the population of the home system's minor world will reach 1-3B and stop increasing, even with a high approval rate. I never colonize these worlds, I prefer to let the AI colonize them and then let them flip, so usually all tiles are full when I receive them. After I terraform a tile and add a farm the population will start growing normally.