<p>My Tevo Flash works well. Right now, I'm trying to print a thin, disc-like part on supports. The 3 perimeters at the edge of the disc are OK. The dense fill zigzag pattern makes many U-turns near the perimeter. It all looks OK in Simplify3D. But in the printed part, in several spots, the zigzag pattern doesn't reach the perimeters, leaving a ~1 mm gap.</p>
<p>My guess: in those spots, the filament has nothing to grab onto underneath, so the U-turn region is dragged back by the nozzle a bit (away from the perimeters) and/or it droops. I'm using the smallest support res in Simplify 3D: 1 mm. Any options I can try?</p>
<p><a href="https://i.stack.imgur.com/nxpz1.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nxpz1.jpg" alt="enter image description here" /></a></p>

The zig-zag pattern are known for this kind of fluke...
I will use Cura to demonstrate, but it's gonna happen on the majority of slicers.
See this example:
Because nothing is really a circle on a cartesian plane and there's resolution limitations on the real world, the slicer has to account for these and, assuming you are using zig-zag to save time, they are doing what is expected. The Zig-zag pattern is a continuous, fast way to fill the top/bottom layers.
Now, see this lines pattern top layer:
It has much smaller air gaps. Every time the nozzle finishes a line, it has the chance to reposition itself to make a much smaller gap, just because isn't continuous.
So yeah, the air-gaps are a "feature" for zig-zag patterns, but you can mitigate this effect reducing the line width on top/botton layers.
It seems that increasing "Skin Overlap Percentage" on cura mitigates this effect too, but i really haven't seen the effects of this setting on a real life object