e.g. 'Mister Potter'
Shouldn't this have been 'Herr Potter'?
Is this a modern thing, English dubbed into German thing or...?
"Mister" is usually not translated, especially not in movies. Same is true for "Sir" (for what we do not have a real equivalent in German; "mein Herr" would come into mind).
The main problem in movie dubbing is to speak lip-synchronously. "Herr" has one syllable, your mouth stays open while speaking. "Mister" has two syllables and your lips close for the "M"-sound. It would be awkward to see "Mister" and hear "Herr".
So if the dubbing people have a easy possibility to avoid this problem, they will use it.
Over the years we got so used to the English forms of address, that probably most books do not translate it either. Out of my head, I cannot say if the translated books I have, use "Mister" or "Herr". But I've picked five books randomly from my shelf and all use "Mister" instead of "Herr". You see, I do not even recognize it any more.
I have neither read the books, nor seen the movies, so I can only guess:
They wanted to keep a British flavour to the original, and therefore stayed with "mister" instead of using "Herr". This is quite common in dubbed movies, e.g. in Matrix the translators chose to keep "Mr Anderson" for agent Smith addressing Neo.
That said, especially movie and TV series dubbing often is bad. The Simpsons in particular are infamous in that respect, and another vivid example is the dubbing of "Alice in Wonderland", that I have seen on TV recently ("Farewell" => "Fahre wohl" instead of "Leb wohl", "wonderful" => "wundervoll" instead of "wunderbar").