So nobody has an idea what could be the reason for the "displayed" security patch level to drop down to May 2022 again?
Perhaps old slot got involved. Flashing always goes to other slot.
You can try to flash from local file (to other slot) and before reboot let Magisk patch that other slot. Or you can reboot and then adb sideload Magisk to current new slot (using for example PE 13 recovery image).
The full OOS ZIP can be downloaded with Oxygen Updater.
Flashing again and applying root as described does not hurt.
Personally I flash both slots with latest OOS, one rooted, one not rooted, so I can, if I need so (for specific apps) temporarily boot an work not rooted.
Most apps can have root hidden, but some not.
In any case, to me it looks Magisk for some reason patched your previous slot.