I get the same thing as MeatyD, but I've only ever messed about with slopes - never implemented them into a finished map. Sometimes they work/appear without a hitch, and other times I get the random squiggles with (or without) the ability to adjust vertex heights. It seems to depend on how I 'cut up' the sectors. I'm guessing there are right and wrong ways to do this.
I just chalked it up to the fiddliness of the feature, and my lack of practice with the slopes.EDIT
(potentially irrelevant to this thread):
Re directional dynamic lights: This is a ZDoom feature-thing, not a GZDB-thing, but maybe it already exists. Is there a way to make a sector that will not be affected by dynamic lights? In this way you could create a dynamic light (say, so it looks like it's coming from a wall sconce), and make a sector in front of it that describes the light's path, like we now do with standard sector lighting. Here's a picture
of what I'm talking about. Would something like this work for you, Numnzel?