I use always that way. The module is called TWRP_A_B_Retention_Script-Magisk.zip.
Today i did a clean flash, followed the instructions in OP. Everything work Well.
I delete the Video by mistake. But here is now the link:
Right, looks like I need to find out what parameter of that backup command refers to the vendor image backup. It doesn't detect "V" as a valid parameter which is why it's not backing up for you.
Just tried again on my 5T, with t...
ui_print("Helix Engine CAF Boost Framework Disabler");
run_program("/sbin/busybox", "mount", "/system");
run_program("/sbin/busybox", "umount", "/system");