Same issue here
Seems that getting an OTA once the tab is rooted is more tricky than expected.
Osiris, try to start your tab with "vol-" and start button and release them just when the Lenovo's logo get on screen.
It's quite uneasy because you have to hold them until the logo appears then release really immediately so you may need many tries.
Once in fastboot, flash again your magisk_patched.img.
i hit the VOL - and the power until reboot (vibration) then i hold the VOL + .
flashed the magisk_patched.img and the tablet boots up normaly.
How can we install the newest OTA ?