Shouldn't this be the case?
// old:
#define XDP_WALLPAPER_TARGET_BOTH (XDP_WALLPAPER_TARGET_BACKGROUND|XDP_WALLPAPER_TARGET_LOCKSCREEN)
//new:
#define XDP_WALLPAPER_FLAG_BOTH (XDP_WALLPAPER_FLAG_BACKGROUND|XDP_WALLPAPER_FLAG_LOCKSCREEN)
libportal/libportal/wallpaper.h lines:42