For some reason sometimes Simplify 3D seems to get confused about what's a fill area an not. It fills in the open area and doesn't put infill in the filled area. The most dramatic I've ever seen the effect is on the attached model and screenshot.
The only solution I've found is to use a different slicer.
I'm using 1 shell, 0.15mm layers. The model needs to be rotated 90 in the X to get it to lay down as in the preview.