In Layout I find that the thickened display of surface edges clipped by a clipping plane often turn off with deactivation of a detail view. Usually (but not always) this corrects itself with the activation/deactivation of any other view. (i.e: activating/deactivating one detail view will cause another inactive view’s clipping planes to redisplay.) This then often causes the recently deactivated view to have the same problem. Attempting to get multiple views to display properly at the same time is like a game of whack-a-mole.
Not only does this happen in render and shaded view, but also in the display of Make2D linework for which the clipping plane layer has been assigned a heavy line weight.
Also the heavy clipped edges sometimes don’t print even if they aren’t displayed in Layout view.