Strange formatting result for string format

When I tried to reformat the code below:
```python
print(\
'Lorem ipsum dolor sit amet, consectetur adipisicing\
elit, sed do eiusmod %s incididunt %s labore' % (str(string1),str(string2)))
```

The **results** are very strange.

First reformatting result is:
```python
print(\
'Lorem ipsum dolor sit amet, consectetur adipisicing\
elit, sed do eiusmod %s incididunt %s labore' % (str(string1),str(string2)))
```

When reformatting again for the result code, yapf reformat like below:
```python
print(\
'Lorem ipsum dolor sit amet, consectetur adipisicing\
elit, sed do eiusmod %s incididunt %s labore' % (str(string1),str(string2)))
```

and again, again, .... it will continuously reformat like this.

Please check this issue. Thank you.