iOS 12.3 Beta 2 - Upcoming changes
With iOS 12.3, the following changes are coming to HomeKit regarding TV Accessory:

- It is no longer possible to select a specific input when the TV is off (see image)

<img src="https://user-images.githubusercontent.com/34152761/55802609-df760400-5ad8-11e9-89b9-b88e662ef4cd.png" width="50%" height="50%" />