I've been finding several immovable irreplaceable empty block spaces in my pre-release world. So far these are in dripstone caves and geodes. I'm not entirely sure what the game thinks is in these spaces but it has become quite frustrating to work around them.

I did use a command block trick to put a "moving piston" in the hands of endermen to prevent them from griefing my world without disabling villager farming and sheep eating grass.  I'm starting to wonder if endermen are able to place these invalid blocks now.