Exactly the same issue here. Samsung QE6FN, Prime video is the only app freezing and hanging. Not every time, but frequently enough for it to become very frustrating. The issue resolves itself after some time (5-10 mins).
This problem seems to be widespread and quite old, so I'm really puzzled why Samsung and Amazon didn't produce a solution... Oh, wait... Is it maybe because they don't care about their customers that much?
For those who can't open the Amazon Prime app or get a perpetual spinner and are therefore unable to sign out of the app.
I was able to take the following steps to get Prime to work again:
I did this on a 2019 Samsung Frame.
Same issue with my Q9FN, it worked ok until recently but is almost unusable now.
Usually when I first select Amazon Prime nothing happens (this part has always happened), I then have to select it a second time for it to load but it doesnt work properly and just displays grey boxes instead of thumbnails so I have to force close it.
When I try to load it again nothing happens, after a few attempts I usually give up and load another app like Netflix, Plex or YouTube and start watching something else, after about 5mins Amazon Prime suddenly loads and closes the other app I am currently using which is very frustrating.
I have reset my TV and logged out but it only fixes the issue once, the next time I try to use Amazon Prime I'm back to square one!