I'm having issued in Plant 3D (2014) with pipe or pipe fittings not staying connected to xref'd equipment. I will model the pipe and connect it to a tank(no drop!) for example. Then at a later time for some reason the pipe will no longer be connected to the equipment (drop comes back.)
Not sure if its related but sometimes a fitting (like a valve or elbow) will do something similar. For example: I modeled a run of pipe and then realized a ball valve needed to be added into one of the lengths of pipe running between two elbows. Without moving any other components i will disconnect one end of a piece of pipe, shorten it up, add the valve, and then try to reconnect the pipe and it will not connect.
The only way i can get it to reconnect is if i delete that piece of pipe completly and redraw it.