I do not rule out that, if you look at the source code or git history of their code, there are comments or test cases explicitly stating they deviate from enforcing a fixed maximum length in code points if doing so breaks up a grapheme cluster.

Also: what do other browsers do when you paste, for example, an ‘é’ that is made up of two code points ‘e’ and a combining character into a field that accepts only a single further code point? Do they just add an ‘e’?