Hey, @jonch!
Thanks for reaching out here!
We're sorry you're experiencing this. We understand that this could be annoying, and we'd like to be able to help.
Have you tried checking if the Seasonal/Sponsored Wallpapers under Settings > Theme, were turned off? If not, you may need to turn it off so your wallpaper will not automatically change.
Let us know if you need more help, we're here to assist.
Best,
The Roku Community Team
Emmanuel-D.
Roku Community Moderator