Bug Get black screen appear for a while when re-open minimized game on Android.

Hello everyone. I'm new to Unity, and I try to build my first game. Today I got a problem when I build my game on my Android device, I minimized my game and reopened it, it got black screen for about 10 seconds. This is video of that bug.
I don't know what happened, and I test my game on 3 Android device (Snap 636, Helio G98, Snap 845) and all of them have that bug. Does anyone have solution? Thanks for reading. (And sorry for my bad English).