Mark Prior Read - Wrong Amount Listed?

Hi,

Just wondering if anyone out there has a fix for an issue with the Mark Prior Messages issue I have?

When I hold the date to bring up this option, and then select it, it comes up with a random number of unread messages. At the moment, as a test, I have one unread message, yet it says "All 1,013 prior messages will be marked as read". If I click continue, it will mark the one message as read (including the phantom 1,012 others) yet next time, it will give me a different figure of maybe 989 message that are unread.

Is there a known issue with this, or a way around it?

Thanks.

Z10 with 10.3.1.1565