Prior to the Orlando Release, chart reports (pie, Multi-level pivot table and so forth), Group by reference field or choice field with the same display value just displayed the field's display value:
From the Orlando release, the same reports that use Group by reference field with the same display value appends sys_id in chart reports
This is similarly seen which choice values:
Release or Environment
Orlando and later.
Was resolved in the Orlando release, and as part of the fix, this behaviour was introduced.
If the criteria of the Group by does not produce duplicate values, the issue is not seen.
Our development department is looking into a way to either make the behaviour configurable, or to document the way it now works.