I found a bug in unreal 4.25.0
If you want to have a splash screen for your Android app, the image will stretch in the first seconds, but after that there will be an image that is no longer stretched for a second and after that you will simply be in the app as usual.
Since this went well in all other versions and I use the same settings, I think this is a 4.25 bug.
As an example for this bug, I used UnrealMatch.
On the 3rd picture you see the bug.
As you can see there are black bars at the 3rd picture and the splash screen is no longer displayed properly.