I don't use OFW but back wallpaper is come from /system/framework-res.apk/res/drawable-sw720dp-nodpi-v13/default_wallpaper.png
Can you try to over set on build.prop
ro.config.wallpaper=/system/etc/default_wallpaper.png
and push from adb your favorite wallpaper on /system/etc/default_wallpaper.png
adb connect <ip_player>
adb root
adb connect <ip_player>
adb remount
adb pull /system/build.prop
add with notepad++ on end of build.prop ro.config.wallpaper=/system/etc/default_wallpaper.png
adb connect <ip_player>
adb root
adb connect <ip_player>
adb remount
adb push build.prop /system/build.prop
adb push default_wallpaper.png /system/etc/default_wallpaper.png
adb shell chmod 644 /system/build.prop
adb shell chmod 644 /system/etc/default_wallpaper.png
adb reboot
If like this not solve then try edit on framework-res.apk, resign with platform key and push back via adb.
Don't forget to make backup, backup, backup because touch build.prop/framework-res.apk can end on loop, then OFW need to reinstall with paper clip.
Enjoy moding of OFW.