Here is how you can reproduce my issue:
1. Create a custom field of user picker for Task Issue Type for designated project, namely "developer(s)". For all existing Tasks, select one user under the "developer(s)" field
2. Create a dashboard, add gadget "Issue Statistics"
3. Settings: Project = designated project; Statistics Type = "developer(s)"; Sort by = "Total", the rest stays as default
4. Save the settings
Instead of showing the count of Task issues grouped by "developer(s)". only "none" and "irrelevant" bars are displayed

If I replace Statistic Types as "Assignee", the gadget will display Task Issues grouped by "Assignee" without problem. Is the custom user picker field "developer(s)" the problem here?