I noticed this bug for a while but it was hard for me to understand it's origin. After fiddling around with it, I realize it's a clipping bug. When the boundaries of the clipping meets my computer screen's edge, it acts as if there was no clipping which can be a problem if unsightly background-ing is held back by the clipping. In this case, I'm playing dm6 and group 4, layer 1 (the only layer) is showing which is indicated by the katana cursor in teh image below.
It's not only this map; it happened in another map when my friend tried to make a dark cave with a dim light bubble around the tee which required clipping. I was the only one that noticed this bug. I'm using the newest version of Teeworlds and my computer is Window's XP.
Give others the respect you wish to be respected with. In the mean time, enjoy the website!