I experienced same issue. What I did to clear lock screen wallpaper (Google Nexus 5x, Nougat 7.0, stock device (not rooted):
1. Backed up my Nova settings in Nova app (to restore later)
2. Uninstalled Nova
3. Google Now launcher became default launcher
4. Pressed down on screen to launch "Wallpapers, Widgets, Settings" choices at bottom.
5. Tap Wallpapers, chose desired wallpaper for lockscreen and then the Google launcher prompted: Change: Home screen Only, Lock screen Only, Home and Lock screen. Chose Lock Screen.
6. Reinstalled Nova and restored settings.
7. It worked.
However, must acknowledge this was the only workaround I could find without extensively searching the web, factory reset, etc.