Hi, I know how frustrating this must have been because it was really frustrating for me since it happend to me as well. I found the solution and i was able to fix it. If you used a different launcher before like Arrow launcher, Action launcher (don't remember which one it was) one of them was resetting the wallpaper. i disabled the launcher and since then the infinity wallpaper hasn't changed. I used samsung smart switch to port data from my old note 4 to Note 8 and i believe it ported the launcher settings as well and i think i tried the launcher on note8 and since it was running on the backgroud it kept changing the wallpaper. I signed up just to post this solution. hopefully it works for you as well.
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