When using tab completion on commands, tab completing any literal parts of command names (like "advancement" or "players" in "scoreboard players") will add a trailing space after the literal.

However when tab completing any non-constant literals (such as player names, "sidebar" in "scoreboard objectives setdisplay sidebar", or objective names) no trailing space is added at the end.

This inconsistency makes it difficult to quickly write commands because of the seemingly random additions of spaces after parts of the command.

The attached pictures were taken immediately after using tab completion for each element of the command.

(On a personal note, if the additional trailing spaces are not a bug, I think it would be better to not have trailing spaces at all since anybody using commands has gotten used to the lack of them.)