When the families are face based, like many of the electrical fixtures, with the families properly done, you still see 3D geometry showing through in the legend. What lile001 suggests, setting the projection lines to white, is the only workaround.
I believe this to be a bug. See image below. The second image shows how the families display in the legend once the projection line has been set to white. The only one that shows that gray square is an out of the box family, which can be done better.
Default legend view
Legend view with Projection/Surface Lines for Electrical Fixtures set to white
The other issue is that these symbols cannot be rotated. Though I am sure they exist, I have yet to ever come accross a electrical symbol legend with the symbols placed sideways. The ones I am used to work with appear as the image below. Currently to do what's shown in the provided image, I can't use a standard legend and drop my electrical fixtures in it. There are many ways to accomplish this, but none as nice as if this was supported directly when creating component legends.
Also, does anyone know how to directly submit bugs to the Revit team within Autodesk? As in, what is the best way of doing so.