I didn't test this with master. Under the covers in stand-alone mode, the "min" function is silently substituted for the grouping, but that's not true in SorlCloud mode. I broke this JIRA out separately to discuss whether it ever makes sense to group by a tokenized text field.
Grouping by the min value in a field is at least consistent, but on a text field I don't think it makes sense.
I propose that we explicitly dis-allow this in both stand-alone and Cloud mode, especially now that there's the SortableTextField.