There's an option in the config to replace the default anime background by a more sober one with the hyprland logo (wish I had know that BEFORE doing a presentation on a large second screen for the first time and realizing that hyprpaper kept my custom background on my workspaces but defaulted to the anime wallpaper on the second screen because in hyprpaper backgrounds are configured per screen 😂 ). And no matter whether you use that option or not, it shouldn't overwrite the background you choose. It's displayed only if you don't have any background configured. Otherwise it's either a bug or misconfiguration in hyprpaper.
Definitely not going to defend the dev on the other stuff.