Hey Michael,
Thanks for reaching out to us!
That is definitely a strange behavior for the plugin. Do you happen to notice wrong dates being displayed anywhere else on the site or in any other emails?
As a first step in diagnosing, have you completed the testing for conflicts instructions to see if there is potential plugin or theme conflict?
Sorry I wasn’t able to give you a direct solution but hopefully we can narrow it down by completing the steps in the linked tutorial. Let me know how the testing goes.
Thanks!