Group By Error

I created a view with the following mappings in in the output schema:

Column 1 :     "VENDOR_BANKING".BANKN_Bank_Account_Number

Column2 :     substr("VENDOR_BANKING".BVTYP_Partner_Bank_Type, 1, 3)

Column 3 :     Count("VENDOR_BANKING".BANKN_Bank_Account_Number)

To enable the aggregation of C3, I obviously want to GROUP BY the first 2 columns.

However, in the Group By tab, there are only 2 options:

> Propose Columns

> Remove

"Propose" adds in all non aggregation columns, but ignores my custom mapping.

So where above, I have selected a SUBSTR of field BVTYP,  the Propose action actually adds in :

"VENDOR_BANKING".BVTYP_Partner_Bank_Type

.
i.e. the Entire column, with no substring.

For the life of me, I cannot see an option to manually edit this mapping !?

Obviously this will produce incorrect output .... can anyone advise if I'm missing something here ?
or is a bug ?

Many Thanks