Exploding short arguments after multi-line string

[moredatarequired]

Operating system: Ubuntu 18.04.2
Python version: 3.6.8
Black version: 19.3b0
Does also happen on master: Yes (tested on black.now.sh/?version=master)

This might be a duplicate of #789 , and/or intended behavior, but I don't understand it.

Input:

msg = """
Message {foo} over three lines.
""".format(foo)

Actual output:

msg = """
Message {foo} over three lines.
""".format(
    foo
)

Expected output:
(no change)

If this is the intended behavior, could you explain the rationale? The style guide doesn't seem (to me) to indicate that black will make this change.

[zsol]

Duplicate of #256.