"Last assignment to resolution time (min)" returns negative time length, cannot work as written

Feature Request Summary: 

Please build a functional attribute for Last Assignment to Solved. The current one cannot work properly due to how the formula was designed. 

Description/Use Cases: 

The metric Last assignment to resolution time (min)" does not work. It will return negative values for assignment time if the ticket was assigned multiple times. This is a fundamental flaw in the base formula. It would need to use some sort of DATE_LAST_FIX to accommodate tickets that have had multiple assignments and multiple resolutions. 

This is the formula as it is currently:
DATE_DIFF([Ticket solved - Timestamp],[Ticket assigned - Timestamp], "nb_of_minutes")

 

This was uncovered while looking at a prebuilt report for the new beta dashboards, "Efficiency: Last assignment to resolution time median [template] ". This report returns bad data out of the box as a result. Here it is narrowed down to a single agent as an example: 

Business impact of limitation or missing feature:

It's extremely important that the prebuilt metrics are functional and return accurate results. It hurts trust in the product when the system is returning inaccurate results from reporting Zendesk built.