Buildings costs nothing to build in certain region

[Jakoporeeno]

I have encountered a bug playing as Pergamon: I own two provinces - Asia (cizycus) and Bithynia (Ancyra and NIcomedia) - and now in Bithynia every building costs nothing to build! This doesn't happen in Asia though. This is what it looks like: https://ibb.co/hsvC0y. I reloaded the turn before it happened and it happened again the next turn. I also have an Oathsworn Urbs and it costs like usual to convert it to a greek building
UPDATED: Okay, I've got a Carthage campaign and this occured again in the city of Hadrumentum in Africa (City of carthage is still fine, though)