Pull request update times are ignored by the updated filter, notice the updated label for search results listing a more recent date than what was requested:
https://github.com/babel/babel/pulls?q=updated%3A%3C2018-02-01+is%3Aclosed+sort%3Aupdated-desc+is%3Apr

Issues are filtered as expected:
https://github.com/babel/babel/issues?utf8=%E2%9C%93&q=updated%3A%3C2018-02-01+is%3Aclosed+sort%3Aupdated-desc+is%3Aissue

We have to use the updated filter to get lockable issues without parsing the entire issue list, because there is no filter for unlocked issues. The updated filter bug causes the app to stop locking pull requests once at least 30 of them have been locked.

I have contacted GitHub support about the issue, will update this thread once we get a response.