This has been happening in all of my games where the AI or sometimes even I will get full influence over a random independent people that I never knew existed. This leads to me or the AI just discovering territories it shouldn't even know about because its half way across the map. For instance in my game that I am running I was the first to discover the new world and the first to unlock Three-Masted Ships yet when I finally found the new world and met my first independent peoples they were full influenced by Gilgamesh(Achaemenid Persians) who had literally just gotten into the medieval era not long ago with 0 influence stars. I then look more at the land and find that he has multiple independent cities annexed that he's probably had since the classical era, I further investigated and played as him to check his tech and he's still busy researching mid classical era tech while somehow owning parts of the new world he physically can't get to without dying. Another instance of this issue I found was I started to influence a a independent city where I was second place at 40 points and Gilgamesh had full, no one else had any influence at all I then look back 1 turn later at the city and see that now Tjilbruke(Haudenosaunee) has just annexed the city despite (a)Not knowing of the new world as she also just hit the Early Modern Era and (b)She had 0 influence 2 turns ago. This bug is extremely frustrating as I love exploring the New World and being the first one there only to find that the AI just knows about the new world and annexes cities with 0 investment other than just hitting assimilate with no progress. Its super disheartening and I feel like just starting a new run every time it happens because if being the first to a tech and actually being on top of exploration means nothing if the ai can do it without either of those investments is just awful.

Screenshots of turn 160 & 161 where Tjilbruke goes from 0 influence to annexing the city in a single turn

bump as it is still an issue

I experienced this aswell in a game I am currently in. I started populating a new continent, and suddenly the AI found it's way there too with no prior interest in going there, and within a turn they had suddently assimilated an independent people on the new continent, with no affiliation with them at all