I can replicate this in SU / Layout 2020 and also 2019.
The problem is that as the section cut plane approaches a wall it is aligned to, and/or as one zooms in to the scene in Layout, the wall in question becomes filled with the cut plane fill material (whatever was set in the scene settings.) This does not happen in SU - in fact, when you double-click inside the model in Layout the fill goes away - only to return when clicking out into “paper space”.
Another interesting twist is that this problem only shows up when looking from a particular direction. Looking slightly “up” at the section cut eliminates the problem.Attached you can see the model and layout files. In the layout I have one view showing the problem, and another showing no problem…
thanks for any help with this!