It's a bug that a player spawn of default or team makes a supply box when the spawn is not used as player spawn in different game mode;

-In DM, a team spawn generates a box.
-In TDM, a default spawn generates a box.
-In TDM, any of spawns generate a player who don't have their team spawn.


And the list below is better I think.

-Player spawn should not make a box.
-Box spawn should not make a player.
-A player who don't have their team spawn can be born at a default spawn (because some of old maps don't have a team spawn and they would be unavailable).

And what if there is no default spawn? I don't think your third point is really a bug... but I hadn't noticed the box spawns before.

I don't know the difference between a bug and function. The point is there's something upside down. So if the third one doesn't work, when a guy who don't have theirs joins the map in TDM, he will be stuck with VvvvvVvv sound as well as anybody can't exist in a map without any of spawns. I think a map with no default spawn is exceptional and it deserves.
And the box spawn seldom happens.