Magisk losing root after reboot on Android 8.1

[angercore]

I downgraded to Android 8.1 as directed by the tutorial - everything is fine.
Then I rooted with Magisk, without TWRP (fastboot flashed magisk_flashed.img with the latest magisk). It works fine (grants permissions to apps) until I reboot. After reboot it says Magisk is not installed and there is no root access.
Please help a noob here, should I try another method, am I doing something stupid? Thanks!!!

 

[angercore]

Fixed it after a lot of searching. I recommend this thread not be deleted because I saw many people searching for this, the answer could help someone and this step was not in the tutorial that I followed.
The answer is here, point 8:
https://forum.xda-developers.com/mi-a2-lite/how-to/guide-install-magisk-device-t3825626