Friend having to download map

[ToxicChilli]

I've notice in Alpha 19 that when i start a game (local to my computer), and my frirnd joins the game he has to download the map everytime.

 

This is behavior we have not seen in any previous Alpha.

 

Is there a way to work around this as it is taking him 15 minutes or so to join the game.

 

Thanks in advance