My television should work in 4K but it does a horrible job of it. So I've always had my Fire TV set 1080.
About 2 months ago, the FireTV started flipping back to "Auto" resolution, switching to 4k. Every evening now, I need to go to the FireTV settings, change the display resolution to 1080 and save. This works for the evening, but the following day the resolution setting is back to Auto.