Making a note for myself because I spent like an hour trying to figure this out...
I'm running LineageOS 21 (Android 14) & just had an OS upgrade. Problem was that I rebooted when the process was done instead of having Magisk flash to the inactive slot, meaning I'm not rooted when my tablet starts back up.
Instead of patching & flashing the boot partition, on the Pixel Tablet you're following the same guide but with init_boot instead of boot.
Just dropping this so I remember for the next time this is a problem!