Alpha channel for color seems didn't get correct value, so you get artifacts when reparent something in "root" window, for example if you do vertical split you get a "copy" of old window without reparenting instead of wallpaper's background. I set background with hsetroot.
Also if I turn off transparent_background in theme file all works well.
As far I know there was some similar bug in awesome. To fix it developers used alpha-channel(32-bit) color for all awesome's "paintings" include "wallpaper"/rootwindow. I hope this info can help.