Parts which are converted as Quilts (surface parts) are not visible when selecting the topassembly. They must be selected seperately to show them in a viewport. This seemed to be 'As designed' behaviour. However this is very confusing because you cannot see the parts when importing the assembly from Step or opening the assembly from Model Manager (same behaviour when deselecting the top assembly!).
Better behaviour would be to turn on visibility for all objects within a structure; because of the different color it's obvious which parts are solid and which parts are surface parts.