Almost fixed it on my Mi 5s Plus [LineageOS 15.1 (8.1.0), F2FS /data and /cache]. Some apps are showing up on "Superuser" tab and never asks for root confirmation again (AdAway, EX Kernel Manager), some apps are not showing up and still asks for a confirmation every single time (Devcheck, Nova Launcher).
I wasn't able to use the latest APK by the way, it was FC'ing. Instead, I used the one from 15.3 (1531).
Did you read this in the op of the unofficial thread:
"Magisk Manager: The snapshot APK is signed with a different key than the release APK so you can't install snapshot magisk manager over magisk manager from Play Store. You will need to uninstall existing Magisk Manager and install the snapshot APK afterwards."