This might seem like a nitpick (well, that's cuz it is), but on top of Mainz being on the wrong side of the river Rhine (ironically stomping on the apparent grave of their Wiesbaden rivals), there's a bit of Hesse sticking out into the Rhineland where Mainz should be. Which makes it impossible to satisfyingly actually extend the natural borders of France up to the Rhine. While it is true that Mainz was part of the Grand Duchy of Hesse for a while, what's the point of making their land extend up to where Mainz should be if it isn't actually there? This is clearly the worst of both world.
Is this extremely minor? Yes. Do they have much more pressing issues to fix, like the inability of the AI to handle the latest patch? Also yes. Is it making the game unplayable? ABSOLUTELY