An OTA update would always install the update to the inactive slot. Since you told Magisk to install to inactive slot in your step 4, the updated boot image is now patched. And it's the patched boot image that gives you root.Maybe i am wrong but regarding my theory if i flash magisk .zip i am patching the boot .img. If i do not do, obviously the updated rom will boot with the updated & unpatched boot .img therefore magisk manager will be there but root itself will be gone. In other words i have to flash magisk again from manager. That is why i am 'patching' again the updated boot .img after OTA is done.
Am i wrong then??
So flashing Magisk again isn't accomplishing anything.