I have both toItemScript and toHandlerScript specified, so this should handle both directions.
As to what UI I’m refering to,I use just the default MainUI, specifically the autogenerated buttons / rollershutter control by the semantic model.
I think it’s worth investigating if the unit is the issue. There’s already been one report of a % unit in a label interfering with the proper function of a widget:
Try removing the unit in the label on the item.
By my understanding this shouldn’t happen: 1) I though the new UoM only applied to Number:unit items, but that’s a dimmer item in the previous report so it’s possible this extends to the your rollershutter item, 2) the arrows in the rollershutter widget are sending UP and DOWN commands, so I have no idea why the UoM should interfere with that.