Try to change the grouping to be for the following:
Current Assignment > User
Or open Object Designer and go to the attribute Current Assigned Analyst and check the calculation that is written there. (You can check the result by adding the attribute into the window and see if it gets updated when the request gets assigned to someone)
Let me know if that worked for you.
Thanks for the reply but 'Current Assigned Analyst' is an attribute directly on the object. I believe if it were a faulty calculation it would not be a sporadic issue but I don't have time to research fully. I contacted Support and Jonathan Peck helped me resolve. All of the queries using 'Current Assigned Analyst' as part of the grouping, I changed to use [Latest Assignment>Users>Title]. It corrected the problem. Apparently the 'Current Assigned Analyst' shows a null value sometimes like if the ticket is closed but I have no idea why it was doing it on some of these tickets. None had been closed previously - just some had no value for no reason. Thanks again.
What I suggested would have got you the same result as well. Anyway, glad you managed to solve it!