Edit: This happens after making a program fullscreen, such as a game or a Youtube video. It is consistent with going full screen.

Not sure if this has to do with the Steam Link software or the build thats currently on the hardware.

After you select your PC and the stream starts, you get the initial message telling you to hold down the Guide button to enable the overlay that has the Link power button, few mouse options, Volume control.

On Build 910, after that first time pop up goes away, holding the guide button to bring it up just freezes control (maybe it does bring it up, and its a rendering bug?) but nothing can be seen or interacted with until you hit the back or guide button again to resume control on Desktop.

Edit: Seems to be after a few minutes, full-screen may affect it but it seems inconsistent.

There does seem to be some similar older threads from years ago but it happens everywhere and not just while focused on one app such as Firefox.