I'm got the same issue, will try this and see if it works for me tooSo I ended up just following the instructions here to flash the chinese rom. Looks like the bootloader did lock from the script here. I had the fastboot issue describe later on, but I didnt have to rerun the bootloader locking steps.
How to install Stock ROM - Xiaoxin Pad Pro 2021 [TB-J716F] I tried installing a GSI-ROM on this tablet, which basically worked except for three/four bugs I coulnd't solve. So I tried to restore the Stock ROM in the meantime and will share my...forum.xda-developers.com
It restored Widevine L1 which was the goal so happy with that.