When I use pick-block (middle mouse + ctrl) to clone a command block with nbt, it shows the last command bar I saved.

How to replicate it:

1. Place a command block
2. Put in a command (I used /say bug)
3. Pick-block + Ctrl the command block
4. Open your inventory
5. Place the command block, don't open it
6. Open your inventory again
7. Drag the command block into one of the other blocks to delete it

If you click on it now, the command is the second to last one you edited and clicked done on for that specific command block, or if it's the first command (like this example), a blank bar.

If you click done on one command block but clone another and do this sequence, it seems like it does the exact same thing, except for the command block you cloned.