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WARNING READ BEFORE PROCEEDING
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After flashing TWRP Stock firmware will no longer boot as it needs a patched boot.img
This is simply done by rooting from within TWRP (Advanced / Root Tools / Magisk root) or flashing any magisk or SuperSU zip
You have on internal storage.

As of now I know of now way to revert the above, So OTA updates will not work, You will always need to manually download and flash then root. Unrooted Stock firmware with patched boot.img could be provided, but to achieve this all you have to do is factory reset and root is gone.. but patched boot.img is retained (device still boots)

For this reason the built in unroot option is removed as it can unpatch the boot.img same applies to Magisk or SuperSU uninstaller zips
so don't use them or device wont boot.

I think the above could be because the bootloader gets locked again after reboot.. nubia need to give us an answer about that though..

it is also very possible that using Nubia Tool v1.1.4 "unbrick option" would restore the device to the way it was before,
making the above warning obsolete.. unfortunately I'm unable to test this as the download captcher code is untranslatable.

Here is the link
https://bbs.nubia.cn/forum.php?mod=viewthread&tid=1118215&highlight=%E5%8A%AA%E6%AF%94%E4%BA%9A%E6%80%A5%E6%95%91%E5%B7%A5%E5%85%B7

Maybe someone can let me know?