If you try to copy a command block with the command still inside (using ctrl + middleclick) you cannot with a repeating or chain command block.

How to replicate:
Place down a repeating or chain command block and write a command inside, now use ctrl + middleclick to copy it with nbt to your inventory. If you now place down the command block item you will see it has no command inside. Repeat this test using an impulse command block and you will see it work correctly.

What it should do:
When placing down the copied command block it should place down a command block of the same type with the same command still inside.