Pie Chart Displays Incorrect Legend When Calculated Using Set Analysis

[cfarquharson]

I have created Tables and Pie Charts using the set analysis formulae for the measures e.g.

Dimension: [TaskName]

Measure: sum({$<[ActivityName] = {'Administration'}>}[TotalPeriods])

The data is coming in from a database where there are multiple Tasks for each Activity, in this form:

(These are the only 4 tasks that come under Administration)

For some reason the Pie Chart legend takes random Tasks from other Activities and presents it on the legend. 

On the picture attached, I have highlighted 2 tasks that are presented in the legend that do not come under Administration. This is happening on many of my pie charts. 

Any idea why this is happening and how to stop it from occurring?

[sunny_talwar]

That is strange, never seen something like this before. Would you be able to replicate this issue in a sample file to check?

[cfarquharson]

I am not sure I know how replicate the problem on a sample.

HOWEVER, I have found a way of getting round the problem. If I change the dimension limitation from Fixed Number = 4 to Exact Value > 0 It only displays positive values.