Thanks for contacting Syncfusion support.
The Grid actions such as Grouping, sorting and filtering can be performed based on that corresponding field. We suspect that your requirement is to grouping the unbound column without mentioned field name of that column. This is the cause of the issue and please refer to the following Help documentation,
Please refer to the following code example for unbound column,
. . .
col.Field("LastName").HeaderText("Employee Details").Commands(command =>
Text = "Details",
Width = "100px",
Click = "onClick"
Note: Here grouping can be performed based on “LastName” field.