Why the ball gets faster then slower after a while

I have found this codepen about Phaser. There is something strange about the ball velocity though.

Why does the ball gets faster then slower after a while after some collisions? Is it normal due to the physics collisions it goes through as it should be? It does not seem normal to me.

Phaser used is this one, version 3.16.2: https://cdnjs.cloudflare.com/ajax/libs/phaser/3.16.2/phaser.min.js