Reproduce:
1. Get any block and place into inventory
2. Switch the place of block by hoverin over it and pressing spare space like 8 would be slot 8 in inventory.
3. Place the block it should dissapear.
Variable factors: creative mode, Custom world:Jungle biome,endermites spawned? I'm not sure what factors could affect it but i thought i might as well list them. It seems to be only on the latest snapshot. Please delete if a duplicate post