The dock at the bottom of the screen jumps above the dotted line, no way to get back in the bottom position without restart it

Restarting the iPad did correct it, but it didn't fix it as it happened again. It seems to happen when I'm swiping up to close an app or game. I haven't been able to re-create it, the dock has jumped like that twice in 2 weeks. Thought it may be a glitch in the latest update, but that's only a guess as it has never happened before the update (ios 11.1.1)