Tested horizon by making a 24x24x1 area and the filler stayed red and didn't do anything

Generally speaking, do you mind opening these on http://mod-buildcraft.com/forums/index.php?board=2.0 ? We're trying to keep the list of open issues to the ones that are confirmed and ready to be worked on. This particular one is fixed in recent NextGen fixes, will get in 6.0.7.