Solid diaphragms created automatically with the infill settings don't actually create solid diaphragms if the infill density is low enough to allow any sagging at all.
A stopgap might be to allow for manual setting of number of layers to create the diaphragm in.
Using bridging parameters to figure out the maximum unsupported area, and then either using the bridging parameters and multiple layers, or automatically creating a gradient of infill density to allow for a solid diaphragm of x layers would be better.
Other slicers have dumb, manual infill gradients. S3D should do it smarter just like it does supports smarter.
