"Custom Settings" advanceReviewDate() doesn't clear date after a review

[jules0x]

In public function advanceReviewDate() the function indicates that it will

 * Advance review date to the next date based on review period or set it to null
 * if there is no schedule. Returns true if date was required and false is content
 * review is 'off'.

I'm finding that in some circumstances*, the review date is not cleared after a review, and the page remains reviewable by the user despite having reviewed the page.

* "Custom Settings" is selected, a user is chosen as the reviewer, and a "Review frequency" is not selected

NB: It behaves as expected when a "Review frequency" is selected.