First, mine is just a fire stick (not 4k). I have put it back in my 203 and I have no problem with a stereo feed to do any upmixing with my avr with a stereo feed (which, by the way, with the Disney trial I'm doing via Amazon, I do not get 5.1 audio, but will be cancelling such due lack of content I'm interested in, but thought I'd try it). My audio setting in the fire stick is DD output/DD+ Automatic.
@snakeeyes did you do Disney thru Amazon or direct with Disney?