def test_something():
    assert foo() == """\
assert [1, 2, 3] == [1, 4, 3]
  
  At index 1 diff: 2 != 4
  Use -v to get more diff"""

Here, the white space on line 2 of the expected value is intentional, yet ruff check --fix deletes it without question.

Or is this actually a bug, and it shouldn't be applying to strings at all?

Workaround

Disable this rule and use ruff format instead, as it doesn't modify the content of strings.