From what I've seen in German C1 and C2 topics, the grammar and vocabulary merge at the highest levels.

In English for example, the word 'have' has an incredible number of structures. C1 and C2 really goes into very nuanced usage and no longer surface level.

In German, natural sounding sentences seemingly break B1 or B2 level rules in these special vocabulary forms (ie relearning that haben has a new grammar), and then learning these special grammars.

Or in other words: Natives don't follow the rules as written at B1 or B2. Learning the special exceptions to all kinds of words is C1+ level. So it's both vocabulary (knowing that words have new meanings), but ALSO grammar because it is complex enough to change the sentence structure.