I noticed while testing other bugs that the commands stored inside of the command blocks in my inventory would consistently vanish since the snapshot was released. After some investigation, I found that this had to do with moving them around in my inventory. This also leads to the "Always Active" being reset to "Needs Redstone".

Steps to reproduce:

1. Place a command block.
2. Set it to repeating and always active.
3. Enter a command (I used "/summon minecart ~ ~-1 ~") into the command block.
4. Press Ctrl + middle mouse button on the command block to pick block with data.
5. Notice that when you place this down, the contained data is the same as when you picked the block.
6. Move the command block one slot to the right (for example).
7. Place it down again.
8. Some or all data that was stored in the command block should now be gone.