Command Block commands replaced with "#####"

When I enter text into the command input field of a command block and close it, the text is converted to a series of pound signs.

Difficult to reproduce: Some blocks are fine, others are permanently non-functional. This is on a world in a realm. Placing a new block down in the same place does not fix the issue.

Workaround:

When using /testforblocks or /clone : The bug would not occur when one puts a leading zero in front of the y-coordinate of the destination field. Example:

/clone 70 50 70 70 50 80 90 050 90

Have not tried: Downloading the world, opening it. Uploading it back to the realm.