When updating my 1.8 pvp map to 1.9 I've discovered a very strange bug. I give a player slowness effect with command blocks, then teleport them and when you take away the effect in any way (with commands, milk or when it runs out) you still cannot move. The strangest thing about this is that I tried to do the same thing in a testing work: first give a player slowness, then teleport and take away the effect, and this time it does take away slowness, but it just doesn't work in the pvp map.
I attached a download to this map so you can see what the problem is. When you log in just press the button ahead of you. I hope you figure out what the problem is since this does not happen in 1.8 and/or any other world than this one.

Update to the issue: This bug does not happen every single time (it's appx. 1 in 2 chance) and has a higher chance of happening if the game a slightly lagging