I’d report the problem to support – – that one just looks weird.
Meanwhile, go ahead and set up each activity that you want to use voice control with on your harmony. You’re going to need those in any case. Everything that you’re asking for is likely possible, but to be honest, it’s harder with fire TV then with almost anything else because it keeps changing the order in which things are displayed. I have a fire TV, and AppleTV, and a Roku, and as far as voice control with Echo goes I like the Roku the best.
But it can be done. It just may take you a while work out exactly which activities you want, which is why you might as well start working on them.
Then if you like, you can just use the echo Ifttt channel with Harmony directly. It’s not quite as smooth as what you can get with the official ST integration, but it works well and it’s really easy to do. 
So you can at least get started with that while you’re working through whatever is going on with the smartthings stuff.
Ignore the second message that is covered in the following thread, because the official integration has changed. But take a look at the first message, using “Alexa, trigger…”
I’ve honestly been surprised that the echo/fire TV integration isn’t better without using harmony. I’m guessing that that’s an improvement will see eventually.