I dont think you can group on text fields at all, not just formula ones

You can, for example,

CASE WHEN {daysopen} > 100 THEN 'A' ELSE 'B' END

can be grouped without issue and daysopen is a standard field on an Opportunity that is an integer type field. This problem seems to be unique to currency type fields (i.e. amount, netamount, debitamount, etc.).