This is a PRO TIP ANNOUNCEMENT. For da people IN DA BACK!
Turn off Matter Casting (Settings>Sound & Display>Matter Casting>Off),
if you're watching/listening to something on your FireTV (Mine is a TCL 55", built-in FireTV) and your sweet child wants to hear, "Do You Want to Build a Snowman?" on our Amazon Echo Show in the kitchen, so he asks Alexa to play it, and your FireTV in the other room you're in switches to Frosty the Snowman from 1969 automatically, as if Alexa knows you want to cry today watching your childhood before your very eyes, and switches it automatically "for you." Thank you, Alexa, you is the smartestest and is a grate lissener.
All jokes aside, I've been searching for, what feels like, WEEKS to get a damned answer to this and nothing had worked and no one had a straight answer. It was Matter Casting the whole time. No more forced auto-switching to whatever my children are desiring.
Just thought I'd share, in case anyone else is quietly raging about this minor (but majorly annoying) problemo.