According to an Amazon forum, the problem may be down to to TVs not being able to play DV content at 50 or 60hz. If the fire stick signal is rejected then the stick prioritises resolution and drops it to 1080. For me, using a 2016 LG TV, and with the Stick set
to auto resolution, only Amazon displays in 4k, but uses HDR, not DV. Disney and Netflix both play in DV, but at 1080p. It's possible to change this by setting the Stick to always output 4k, but this will result in Netflix and Disney playing HDR, though it will be 4k.
Swings and roundabouts, as ever, it's a compromise. If Amazon updated the Stick to support DV at 30hz that would solve the issue apparently.
Quick edit, just occurred to me that YouTube also only plays at 1080p but that always uses HDR, so maybe there's more to this? Why does Amazon play in 4k HDR but YouTube maxes out at 1080p with HDR?
But irritated, I thought id found the perfect streaming stick, but not so sure now. I might just force it I to 2160 resolution and go with HDR instead of DV.