You can’t lock the bootloader if you rooted your phone.
I am by no means an expert in AvB, that’s why I’ve called it a minefield, but since rooting your phone does alter your boot partition the hashes wouldn’t match anymore and hashes that changed are no longer verified 
You have two choices:
- If you switch to stock (unrooted) and relock the bootloader you don’t have to unlock the bootloader to perform OTA updates, but you also won’t be able to use root.
- If you want to stay rooted you’ll have to disable Magisk every time there’s an OTA update: