Using the new Grid grouping feature in 1.6, the Grouping button and the group header row both use the field name of the group column, rather than the title.  This is visible in the animated demo, where a column titled "On Vacation" becomes "IsOnLeave" once it is used as a group name.

I expect this is already known and slated to be fixed, but there aren't any grouping issues at all in the public tracker, so just in case...