/execute does not execute as a Player (or entity)

if you want to execute for example the command

/execute @r ~ ~ ~ scoreboard teams join Team 4 

it will give you two error messages:
Player 'commands.generic.player.unspecified' cannot be found
and
`Failed to execute 'tp ~ ~1 ~' as <Player name>
where <Player name> is the @p player.
There are three bugs here, but if one gets fixed, the others become irrelevant:

• The command doesn't work like it would if it got executed from the player (which execute should simulate).
• The player name doesn't get put into the first error message. If the target selection for example meets an item, it gets displayed the same way as in the /kill feedback, but not for the player.
• There are two error messages for one error.

If the first one gets fixed, the other two get irrelevant.