I do not yet use Magisk -- what happens if you forget to do that _before_ rebooting?No need for fastboot. Go into the Magisk app before installing the OTA, click on Uninstall Magisk, then Restore Images. Now install the OTA via the built in updater. Once the OTA update is installed, do not reboot. Go back into the Magisk app. Click on Install Magisk, select Install to Inactive Slot (After OTA), and only then reboot.
Full instructions can be found here: https://topjohnwu.github.io/Magisk/ota.html
The link is equally silent on this, probably not that implausible, scenario.