/fill only supports block entity tags for one of the filled blocks

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:

1. /give yourself a command block (/give @p minecraft:command_block)
2. Paste in the following command and set the command block to 'Always Active':

   /fill ~-1 ~2 ~-1 ~1 ~2 ~1 minecraft:command_block 0 keep {auto:1b,Command:"/say This should only trigger once."}
   

   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.

Notes:

• 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.