I didn't see anywhere that you were calling the code to set your visibility. "Hidden" text is a poor method, because some people like to show hidden text in documents. Put this is the ThisDocument module:
Private Sub Document_ContentControlOnExit(ByVal CCtrl As ContentControl, Cancel As Boolean)
If CCtrl.Title = "CCDDL" Then
With CCtrl.Range
Select Case .Text
Case "Issues found"
.Font.ColorIndex = wdRed
Case Else
.Font.ColorIndex = wdBlack
End Select
End With
'UpdateTextBoxVisibilityI
'Or
'UpdateTextBoxVisibilityII
'Or
UpdateTextBoxVisibilityIII
End If
End Sub
Sub UpdateTextBoxVisibilityI()
If ActiveDocument.SelectContentControlsByTitle("CCDDL")(1).Range.Text = "Issues found" Then
ActiveDocument.SelectContentControlsByTitle("CCTB")(1).Range.Font.Hidden = False
Else
ActiveDocument.SelectContentControlsByTitle("CCTB")(1).Range.Font.Hidden = True
'Note: if you or your users has File>Options>Display>Show Hidden Text checked then you will see the text with a dotted line under it.
'You might want to use as white font, or set the placeholder text to a zero length string.
End If
End Sub
Sub UpdateTextBoxVisibilityII()
If ActiveDocument.SelectContentControlsByTitle("CCDDL")(1).Range.Text = "Issues found" Then
ActiveDocument.SelectContentControlsByTitle("CCTB")(1).Range.Font.ColorIndex = wdBlack
Else
ActiveDocument.SelectContentControlsByTitle("CCTB")(1).Range.Font.ColorIndex = wdWhite
End If
End Sub
Sub UpdateTextBoxVisibilityIII()
If ActiveDocument.SelectContentControlsByTitle("CCDDL")(1).Range.Text = "Issues found" Then
ActiveDocument.SelectContentControlsByTitle("CCTB")(1).SetPlaceholderText , , "Enter issues here"
Else
ActiveDocument.SelectContentControlsByTitle("CCTB")(1).SetPlaceholderText , , ChrW(8203)
ActiveDocument.SelectContentControlsByTitle("CCTB")(1).Range.Text = vbNullString
End If
End Sub