Black keeps this code as is:

def some_function(
    string: str, /
) -> ReallyReallyReallyReallyReallyReallyReallyReallyLongName:
    pass

But Ruff 0.1.7 forces the argument list to be split across lines.

def some_function(
    string: str,
    /,
) -> ReallyReallyReallyReallyReallyReallyReallyReallyLongName:
    pass

I would also consider Black's behavior to be more desirable here. This is a regression from Ruff 0.1.6. I suspect that this was caused by #8921, where it would be easy to think this function has one argument. Semantically, it does, while syntactically, it has two.