If a command block is filled to the max amount of characters, auto-completing the command at the start deletes characters at the end to make room for the name of the command.

I tested with the first 32,495 numbers of pi from https://www.piday.org/million/ and with a /say command.

When I had

/say <32,495 numbers>

in the command block and went to the start and deleted the command up to the / and selected a longer command, (such as /defaultgamemode) the characters at the end of the command block were cut off to make room for the command name.

I have attached a .txt file with the /say command and the 32,495 digits.

Expected behavior would be for the text at the end to not get deleted and for some error to be presented instead as to why the command name couldn't be inserted.