Oh, thanks for reminding me. I remember this having been reported earlier but apparently I forgot to make or lost my note for it. I reimplemented most of of the resources logic so this seems to have fallen through the cracks when checking if everything works. Unfortunate. It's easier to track down with a save. If anyone encounters this again, please upload the save. This is intentional. Nothing special is really going on here. A civ's core stops being considered for "foreign" once the civ cannot respawn again.