I'm using Unity 2018-.2.18f1 personal.
If I create a new app, add some UI elements (panel, text, buttons) and export the project to a mobile device (iOS in my case) the splash screen with show then disappear, then there is a slight delay with the UI popping up.
There is nothing special about the UI, or additional scripts attached.
This is quite annoying, and my fix is to ad a 'cover' GameObject that fades out after 1 second. So the cover object is black and covers the disable screen, when the splash screen finishes you see this black cover and then that fades out and by that time the UI has populated.
The issue with this is that it feels super hacky and I just feel like there has got to be something wrong with my setup or a better preferred method for dealing with this.