When You Shrink The Barrier On A Timer, If You Are On a Singleplayer world and you pause, the world border still shrinks and deals damage.

How To Recreate:

1) Create A New Singleplayer World With Cheats On

2) Run /worldbarrier set 1 10

3)Pause The Game For 10 Seconds Or More.

4) Unpause The Game.