see link for demonstration, you pick custom race, disable custom races, move playercount slider or leave and reeneter your singleplayer lobby from menu and it changes to Nakalim.

However, your custom race remains chosen even if they are disabled.

https://www.twitch.tv/videos/489306270?t=00h00m31s

I suspect this has been persistent in the game probably longer than just in the preview I just never got it documented.