On my dutch test-runs, the burgundian Anvers caused my initial ground stack to be pushed far away, it's between germany and poland.
The game displays my initial starting area as dutch culture though.
Save attached
Also, on 2 occasions I had seen plagues spread out in almoust all of Europe. Most civs must have had it for hundreds of yours, had very few units, and all their cities had only 1 or 2 population.
It seemed that a few lucky civs were immune to it, and they flourished of course. The rest of the civs being so weak, they could have conquer any territory they wanted to.
No save here, but unfortunately it's quite common that the plague mechanics break. And it may cause the entire game to be ruined, as it was in these 2 dutch starts.
Cannot be sure about the exact details, but there is a good chance it is related to conquering plagued cities
My best guess is that this happens (a civ gets total immunity or causes never stopping plagues throughout Europe) when you don't have plague (already out of it or not spread to you yet) and conquer an indy city with it.