From reading other forums it appears that picture quality on Amazon Prime is much better if you continue watching using your Firestick rather than using the V6 app. The frame per second on the Firestick for the UP Open Tennis is 50 compared to 25 on the app. It is very noticeable.
My belief is that the Fire TV Gen 3 has the default frame rate of 59.94fps. All UK and European TVs support both 50Hz and 60Hz refresh rates so 59.94fps is easily handled by TVs which refresh at 60Hz. It makes no sense for 59.94fps to be changed to 25fps for a refresh to 50Hz.
I have exactly the same problem it worked intermittently for a while. It works ok on my upstairs V6 box but not my downstairs V6 box that is connected to my main TV. I tied rebooting it and now none of the apps or games work.
Amazon prime works fine on my iPad and computer just not on the main V6 box very frustrating