I’m finding that every time I do a boolean intersection or a trim the objects that are getting trimmed display the isocurves (the isocurves are present before the operation).
Before Boolean Operation:
I extrude a cube them copy it, explode & re-join to get a polysurface. In wireframe mode, on the left you can see the isocurves in the polysurface, but not in the extrusion. On the right I do not see the isocurves on the polysurface because I turned them off.