did you enable on notification show on lockscreen? some time some app will enable it by default but few app must enable it manually. Just check and make sure enable it in app. and test it.On my Mi6, no notification appear on lockscreen
May be a TWRP version : https://xiaomi.eu/community/threads/twrp-3-2-3-treble-support.47492/My problem is same as yours. Hope someone can explain and show some guides to resolve.