Consider the following simple snippet:

def func(a: int) -> int:
    """A docstring.

    args:
        a: an integer.

    returns:
        An integer.
    """
    return a

This triggers a D405 warning on line 4 -- it's unhappy that args: isn't Args:. However, it does NOT trigger a similar warning for the returns: section. If we get rid of the Args section then a D405 error is reported.

def func(a: int) -> int:
    """A docstring.

    returns:
        An integer.
    """
    return a

I suspect that this is a bug (I wouldn't expect the presence of one section to have any impact on whether that section should be capitalized).