When you attempt to autocomplete the dataTag argument for a /fill command, nothing happens because the game cannot autocomplete a dataTag.

However, when using /fill replace, instead of a dataTag, the person attempting to execute the command may input a block id which will make sure only that block will be replaced. This argument is put in where dataTag would usually be, as an option instead of dataTag.

Because attempting to autocomplete a dataTag argument gives useless results, when autocompleting the argument after a replace mode, the game should give you block ids, putting the minecraft: like it does when autocompleting the TileName argument in the same command.