MIUI 9.6.1 update on my Xiaomi Mi5 phone brought the Android version up to 8.0 Oreo.
The Microsoft “Next” lock screen app no longer works, even after double checking all the relevant permissions – allow overlay drawing / allow popups etc. The only way to make it appear is to disable the system screen lock completely, but this also disables the fingerprint unlock feature.
It would seem that in Oreo, the security model was tightened up, and now apps are not allowed to draw over the status bar using the SYSTEM_ALERT_WINDOW permission. See this post for a fuller description of how lock screens, and apps simulating lock screens, worked prior to Android 8.
This is a real shame, because prior to this update, I was getting good visibility of upcoming events, and better notifications than those provided by the MIUI software. I miss these features. I hope Microsoft will find a way to make it work, but based on the above article, it sounds like this app might be confined to Android versions lower than 8 only.