I think the most jarring thing is when you have a US scriptwriter for an English character, and they end up using grammar or vocabulary that is so obviously not how an English person speaks. Like using ‘gotten’ instead of ‘got’…

Yes!  Totally agree!  I work so hard to keep my American characters sounding American, and the same when I write English characters and it really is the most jarring thing to hear the slang in ‘vice versa’ accents.