I don't think mutability is wrong as a concept, albeit I enjoyed learning and toying with Haskell, but I really think these functions in Python should have been two sets of operations, one set to do what a += b does and one set to do what a = a + b does.
I've been doing C++ for quite some time and amount of implicit things that happen magically and not everyone get them right and this leads to bugs and confusion had really grown on me with time, and I feel like this is the same pattern here, where we get implicit magic instead of being clear with intentions and results