I came about a project where the architectural model had several break lines throughout the project. When anything MEP was drawing over-top of the break line the item would be masked out. The way to fix this is to go into the linked model through visibility graphics and switch the link from "By host view" to "Custom". You can then go into the Model Categories tab and turn off the "Detail Items" by unchecking the check box. Click the OK buttons to get back to the drawing.