1.12.2 deletes commands from commandblocks

If you open an old world, which has commandblocks in it, in Version 1.12.2 everything seems to be fine. But if you reopen it in the old version, the commands are deleted.

What I expected to happen was:
The commands stay in the commandblock

What actually happened was:
They just disappeared

Steps to Reproduce:
1. Create a Minecraft world in a version lower than 1.12.2
2. Place a commandblock with text in it (does not Need to be a command)
3. Leave the world
4. Open the world in 1.12.2
(4.1) Commands are still there
5. Leave the world
6. Reopen the world in the original version
-> Commands disappeared