I also own a 4800 Ultra and while I don't have this specific problem, when I click on a title to read details and press the BACK button, the app returns me to an entirely DIFFERENT category rather than back to where I was browsing. In some cases, the normal categories on the app are completely missing so I have to keep pressing back to get to the landing page. Worse yet, there's NO way to contact Prime video other than leaving feedback which is useless. Roku support should contact them about these issues don't you think?
Power User (film maker)
Mill Valley, CA
Roku Ultra 4800R; LG OLED55B7A, ONKYO TX-NR636, Panasonic Blu Ray, Definitive Studio Monitor 450 (L/R), Definitive LCR 2000 (center), Definitive 10" sub-woofer, Definitive surrounds.