yapf adds spaces before backslash with every run

Let's say I have this code fragment (note the backslash):

```python3
plan_bundles = {
plan_bundle.bundle_name: {
'tier_change_options': \
bundle_util.get_tier_change_options(plan_bundle, details_by_bundle)
} for plan_bundle in plan.plan_bundles
}
```
If I run yapf over that file it will add two spaces before the backlash. Result:

If I run it again:

So yapf is never happy with my file code style. Is this a bug?

Style config:
```
[style]
based_on_style = google
indent_width = 2
column_limit = 100
continuation_indent_width = 4
each_dict_entry_on_separate_line = true
allow_split_before_dict_value = false
split_before_logical_operator = true
spaces_before_comment = 1
blank_line_before_nested_class_or_def = false
```