Now, I am 99% sure that this is a bug, but maybe I'm the one doing something wrong.
Basically, three times in a row now, on different playthroughs, I could see hidden cities visually, but simply wasn't able to actually land at them. All of them look "normal", at least 750km away from the nearest city and surrounded by a mess of roads. However, I can't land, even if my ships are directly above it. I've tried both with my flagship, skylarks, gladiators, whatever, nothing works. The last time this happened, I even had info from a caravan about there being a hidden city around where I was looking for, I found it, but again, can't land.
Is there a fix for this issue? Thank you all in advance lol
Edit: it does seem to have been a bug. I verified the files and the problem has fixed itself, thank you all for your suggestions and help! <3