I had that Amazon app issue on my HU8550. I reset Smart Hub and the problem hasn't returned. It was after an update by the way.
UN65F9000AFXZA + SEK3500u/za firmware version 1461
Denon AVR-X7200WA, Apple TV, UBD-K8500, PS4, Xbox One, Nvidia Sheild Pro, PC
(2) Klipsch RF-82IIs, (4)RS-52IIs,, (1)RP-450CA, (4) RP-140SA, (1) SVS PB12-Plus
Emotiva XPA-100, XPA-200, Emotiva A-700
UN55KS9000 firmware 1132,Denon x4200w, Apple TV, Panny BDT460, PC,
Nvidia Shield, Klipsch G-42,(2) R-3650-W-II,(4) RSL-C34E, SVS SB-1000, UPA-200