I was going to test its ability to matrix decode Netflix' stereo mis-advertised as 5.1, but we've reached season 3 of Peaky Blinders, which actually has a real 5.1 soundtrack. I'll have to go back and test with an older episode later.
If it can't matrix decode this case, I can probably work around it by forcing the Fire Stick 4K to stereo (by restricting audio format to PCM).