There's currently a bug with VLC for iOS that causes the Touch ID prompt to display while attempting to switch to other apps. Here is my version info for VLC for iOS:
Version: 2.7.5 (275.2) All Along the Watchtower
Based on 3.0.0-git Vetinari
Settings:
Passcode Lock: On
Allow Touch ID for Unlock: On
Most other apps using Touch ID don't ask for the touch prompt unless the app is active and the user has the app in focus. As far as I know it's only VLC that's been doing this.