In 4.2 if I group a grid by a column that has a convert function in the model the grid will not render correctly when grouped.

I have this convert function in my model:
Code:
{            name : 'someDate',
            type : 'string',
            mapping : 'some_time',
            convert : function(value, record) {
               return Ext.util.Format.date(Ext.Date.parse(value, 'c'), l10n.dateTimeFormatStrings.shortDateFormat + " "
                     + l10n.dateTimeFormatStrings.shortTimeFormat);
            }
         }
If I group my grid by this column the value in the convert function == "". The record.raw == undefined. And the record.data.someDate = "". There is never any non-"" value passed to the convert function when grouped.

So, here is the grid pre-grouping:
gridungrouped.png

Here is the grid while on a breakpoint in the convert function where the value is ALWAYS "". Interesting that it renders correctly BEFORE processing the null record values...:
groupedonbreakpoint.png

And here is what it renders at once it has processed the convert function:
groupedbroken.png

Now, I could avoid doing the convert here and do it on the renderer function of the column in the grid. The problem with that is that the grouped header then has the UNCOVERTED value displayed. So the value in the grid and the value in the grouped header is not the same?! If there is at least a way to CONDITIONALLY render the groupHeaderTpl that would be an acceptable workaround.