it does it consistently, its really not the fill-tool that's bugged. the program is not identifying colors correctly.
so if you use paint brush 20 size or so, and click in a few random locations, if you use eye dropper and check the value it will slightly alter the color value in random places almost as if you put some noise on your brush - but its 100 hardness and 100 opacity.
man what's with all these bugs!? its crazy how many issues I've had at since this stable update r234. this is like the 4th bug! I keep losing time researching these things!