If you use the /fill command to fill in blocks with a block entity tag (like command blocks with commands inserted), only one of the filled blocks actually has the tag.
Steps to recreate:
- /give yourself a command block (/give @p minecraft:command_block)
- Paste in the following command and set the command block to 'Always Active':
Because of the bug, it will say the message only once instead of nine times, and only one of the nine command blocks has the command inserted.
- I used a similar mechanism in 17w17b, and this didn't result in the issue.
- The behavior is the same when the command is executed from chat or through a function, executing it from a command block was just an example.