This seems to be an ongoing issue with the 4K Fire products. I have the pendent. It works for a while, then it updates and it no longer works. Netflix was not supposed to be working with Atmos, as it's only available with a few OLED TVs, a Japanese only streaming TV box, Apple TV 4k box, Windows 10, and 3 of the X Box models according to their website. As of today, it's in the No Atmos Mode, and I'm ready to just throw it away. I only bought it for the Dolby Atmos sound, and I wish I had never wasted my money on it. I'm sticking with Roku. It doesn't pass Atmos on Netflix, but at least it has the Vudu app, which Fire products don't even offer, and Atmos DOES work with it.
Having the same issue on my Fire TV Stick 4K, updated to latest firmware. Onkyo NR575 receiver. Doing all of the things noted above (watching the UHD 4K version of "Jack Ryan," waiting a few seconds, trying to manually select "Dolby Atmos" audio source but only "Dolby Digital+" and "Stereo" are available.) One note: my TV only supports 1080p video, but i'm setup for 5.1.2 Atmos audio.
Contacted Amazon Support today, they indicated that this is still a "known issue." Folks who are having the same issue, please update the thread if you get a resolution/update.
Yes, have the same issue. Cube, when set to my Atmos Soundbar, does provide Atoms option in menu and soundbar and does switch to Atmos.
When I use the Fire TV 4K it only shows Dolby + in the audio menu when watching the movie. However my sound bar does swith to reflect Atmos when starting the movie. So looks like the Fire TV 4k has a firmware glitch in the menu, but is passing Atmos.