Thank you for your sample.
To make sure the changes you are directly setting in the checkboxclick event are saved, try setting the grid.Binder.DirectSaveCellInfo property while you make the change.
Private Sub _grid_CheckBoxClick(ByVal sender As Object, ByVal e As Syncfusion.Windows.Forms.Grid.GridCellClickEventArgs) Handles _grid.CheckBoxClick
Dim modulo As Integer
Dim newCheckValue As Boolean
''Check box does not reflect change from click yet, so we have to invert it''s value
newCheckValue = Not CBool(_grid(e.RowIndex, e.ColIndex).CellValue)
Me._grid.Binder.DirectSaveCellInfo = True
''checkboxes are always in pairs, when one is checked, it''s counterpart must be unchecked.
If newCheckValue Then
modulo = e.RowIndex Mod 3 ''Each set of data series is 3 rows, Actual, Trend and Blank
Select Case modulo
Case 2
''This is the trend line. Line above must be false
_grid(e.RowIndex - 1, e.ColIndex).CellValue = False
Case 1
''This is the actual data series. Line below must be false
_grid(e.RowIndex + 1, e.ColIndex).CellValue = False
End Select
End If
Me._grid.Binder.DirectSaveCellInfo = False
End Sub