I cannot switch back and forth between streaming stick and cable TV on a Samsung smart TV about 4 years old. Cable is on HDMI 1: Roku stick is on HDMI 2. I use "source" but once I am using the Roku device the TV will not respond to the FIOS signal when move back to HDMI 1. I must pull the stick, unplug power and reboot. My older TV does not have the problem. The stick really screws up my TV.
When you switch back to HDMI1 (FIOS) do you see nothing at all? Possibly the FIOS box has gone to sleep after inactivity?? If so, can you wake it up?
Or possibly you mean that if you press a button on the FIOS remote, then the TV switches back to the Roku input? If so, on your Roku, try going to: Settings -> System -> Control other devices and uncheck 1-touch play.