My Panasonic TX-55EZW954 recently got a HDR10+ upgrade with Firmware version 3.231. I'm using the Fire TV 4K stick with the latest available software version.
While playing HDR10+ content on Prime Video (like The Tick or The Boys) the display does not switch into the HDR10+ mode. The debug view shows "Color Standard: BT2020_10_HDR10Plus", but the display is staying on standard HDR. Also the Fire TV menu always stays in the same standard HDR mode.
On the other hand the display mode is switching correctly when using a side-loaded Kodi, using a HDR10+ test file, on the same stick.
So I assume there must be something wrong with the Prime Video app, or the Fire TV system, detecting the HDR10+ availability correctly?
The attached screenshot shows the problem, with active HDR10+ the logo should be visible in the HDMI info screen from the TV.