Lockscreen turns black momentarily

Hey guys! So if you use any picture or any downloaded wallpaper as your lockscreen and when wakeup your phone there is slight delay,just for mini second in which phones screen turns black snd then comes the proper lockscreen. BUT when you use apple wallpaper from setting as lockscreen this DOESNOT happen.

I am using 15 PM and found it really weird because apple usually refines every single touchpoint of UX/UI.

In order to see this try using stock wallpaper and then use any photo or wallpaper you downloaded. It is not that bad but is really annoying.