It appears it was indeed just a balancing on yields. Some small iterative changes to other areas (like a small increase in desire for science) must've pushed it over some border where suddenly no one was interested anymore. It probably happened in one of the later changes before the release, and I didn't notice because I don't look at district counts every game (and cant see gpp counts in the AI only games). Just turned their desire for culture up a little again and all was fine. In addition I also added a bit more of an extra desire for 'cultural civs'. In vanilla cultural civs were defined, but they had no extra preference for culture/tourism directly, they just had a preference to selecting the culture victory, which would then increase their desires. I think it makes more sense if they have a small base increase in desire too. Yeah I'm not sure what exactly they're supposed to have done that makes it better, it barely seems to make any difference. It's probably a tiny behaviortree change, since that's where most of the escorting logic takes place. It's so tiny I can't spot it though (wish I had saved the original somewhere). And since the whole behaviortree system is so limited, I doubt they could even make the behavior perfect with changes there.