Lots of people have reported issues with gaps in walls. The simulation may show 3 passes, but if the slicer is telling the extruder to back off on the amount of material being extruded to fill a narrow gap, I don't think the preview has any way of indicating that. If the calculation for how much to reduce the flow is off, you
could get the indicated behavior.
I spent a lot of time wrestling with similar issues, and did all sorts of tests. In my case, as soon as I turned on
any negative Extra Restart Distance to get good seams, I got gaps in my perimeters. I experimented with all of the other various gap fill settings, to no avail. Here are all the gory details:
viewtopic.php?t=16041
I passed this info along to Tech Support, but with no updates expected within my lifetime, I occasionally have to switch to PrusaSlicer. It has the same problem sometimes, but usually on different prints. If I have a problem in S3D, I try PS & vice versa... Usually one or the other will do an acceptable job.