The bug
In some commands, the wildcard is a valid value for entity argument, but it's not suggested by the command auto-completion.
Examples:
scoreboard players add
scoreboard players set
scoreboard players reset
scoreboard players remove
scoreboard players enable
team join
team leavescoreboard players add
scoreboard players set
scoreboard players reset
scoreboard players remove
scoreboard players enable
team join
team leave
Still in 18w50a.