Apparently, the new pre-release doesn't like slimes.

How do I know that? Well, one reason is that gravity hardly affects them. The other is that they teleport to each other.

When slimes jump, they either jump in slow-motion, ascending and descending in the speed of a falling chicken, or they just don't fall. They just jump up into the air and travel off into the distance.

Besides that, when a new slime is summoned, for some reason all the slimes in the area feel a need to teleport straight to that slime and mounting it.

I have no command block or anything like that that could cause these bewildering actions. I hope both the problems will be fixed soon.