It happens 3 times in about 2 hours in a multiplayer game that I drowned in water (2-3 blocks deep) because I couldn't swim upwards (there were no solid blocks above me at all...)... the space/jump key did nothing at all. I could walk in any x and y direction, however, just not jump/swim upwards (z). Restarting Minecraft did not change the issue, restarting the server didn't help either. However, after I drowned and respawnt, jumping and swiming worked the way it should again. This isn't reproducible (or at least I couldn't) because I don't see a pattern in it. It doesn't happen on the same places or at a sprecific block type or anything like that. However, because I never had such problems before this pre-release, it could be releated to some of the newly made changes. This issue may or may not be because of glitching into some block, however, it happend only underwater and I was not able to remove the block under me to test this, because after restarting the game and the server, I just hadn't enough time to do so before I died.