Working As Designed.  Been that way for a long time. Talked about this is a session on Custom Office at the Sage Summit once. Bring it up with Steve or Elliott

You need to call the UI object's FormatRow method but it requires a numeric handle to the grid and a numeric value for the row number. In a column level validation event, it is safe to use the CurrentRow property of the primary grid but in a pre-write event, it gets tricky because the CurrentRow property is changed to the destination row before the pre-write fires so you may need a post read script to set the current row into a storage variable of the detail's script object.

Here is a sample if this was a column level or button script.

If oScript.UIObj > 0 Then
	Set oUIObject = oSession.AsObject(oScript.UIObj)
	oUIObject.InvokeChange "PromiseDate$", "20201228", "GD_Lines"
	nGD_Lines_Ctl = 0 : oUIObject.GetControlProperty "GD_Lines", "Ctl", nGD_Lines_Ctl : If IsNumeric(nGD_Lines_Ctl) Then nGD_Lines_Ctl = CLng(nGD_Lines_Ctl) Else nGD_Lines_Ctl = 0
	nGD_Lines_CurrentRow = 0 : oUIObject.GetControlProperty "GD_Lines", "CurrentRow", nGD_Lines_CurrentRow : If IsNumeric(nGD_Lines_CurrentRow) Then nGD_Lines_CurrentRow = CLng(nGD_Lines_CurrentRow) Else nGD_Lines_CurrentRow = 0
	If nGD_Lines_Ctl <> 0 And nGD_Lines_CurrentRow > 0 Then 
		oUIObject.ShowRow nGD_Lines_Ctl, nGD_Lines_CurrentRow
		oUIObject.FormatRow nGD_Lines_Ctl, nGD_Lines_CurrentRow
	End If
	Set oUIObject = Nothing
End If

So as i expected, the pre-write event behaves differently, only way i could get the desired results was with the following.

If oScript.UIObj > 0 Then
	Set oUIObject = oSession.AsObject(oScript.UIObj)
	oBusObj.SetValue "CommentText$", "Test"
	oUIObject.SetVar "CMD_STR$", "X _obj'FormatRow(GD_Lines.Ctl, cChangedRow)"
	Set oUIObject = Nothing
End If