am I understand correctly you’re having another APP partition for your customize system image?
there’s no APP-BK keyword in the script, you may also check the flash script file,
for example,
if [ "${target_partname}" != "" ]; then
...
case ${target_partname} in
...
APP) target_partfile="${localsysfile}"; ;;
please add your APP-BK options into the script file.
you may also search for the APP_TAG to revise the script file correctly.
thanks