The procedure is:
* Unroot using the Magisk Manager app's "UNINSTALL" option (bottom of main screen), but DO NOT REBOOT. This restores the boot partition so that the update can be installed.¹
* Apply the update. It takes a while, especially if it is "optimizing apps". DO NOT REBOOT.
* Go back into Magisk Manager and install Magisk again, selecting "Install to Inactive Slot (After OTA)".
* Only then should you reboot.
¹ The update only applies against unmodified partitions and will checksum them before applying.
Thanks a lot... I figured out that was the problem when I posted on a more appropriate thread... But still, I thank you a lot...