OTA, because I am too lazy for sideload unless absolutely necessary.
Start the OTA, let it grind on ad nauseam, wait for the message to reboot, stop there and open Magisk app, select install to vacant slot, Magisk runs its little script, then swap back to the OTA and touch reboot.
Done it a couple of dozen times like that, it always works.
I recall that you and I installed Magisk in different ways, and had words here about it. I can't recall exactly what the difference was now, but you could parse our old comments here to check it. I think you claimed you didn't need to do this