Simplify3d has always done this - well from version 2 onwards anway.

this is 100% infill:
s3d-100%infill.JPG


And this is with solid diaphragm every 1 layer ticked:

simplify3d-solid-diaphragm.JPG



These settings for both - just 'solid diaphragm NOT ticked for the first image.

infill settings.JPG


Are you perhaps getting infill mixed up with layers and shells ?