That's something to submit to Support so they can improve the Fortigate connector so it parses the data correctly.
Can you open the Reports Console, and run the Tool Maintenance by Alias Report for the last 72 hours? Save it as an RPT, Support will want that.
Have you opened up a support case? Unmatched data can happen because not everything has been seen. They can help get things moving for this event to be normalized.
Assuming that is what you are saying about making it stop.
If it is more about making the event itself stop then I would lookup on the Fortigate site the information about this message.