Article publication date has reverted to today's date on several issues of a journal

One of our journal editors has alerted us to a problem where the article publication date has reverted to today’s date on several issues (on first glance it looks like the last six issues but not the first few articles of the latest issue).

All of our sites were recently upgraded to OJS 3.1.1
Other sites don’t appear to be affected by this. Only this site:

The issue may be related to this: