We had a view where we had the surface pattern turned off in the Visibility Graphics Overrides for Structural Framing for some of the concrete beams. Eventually we noticed that the moment symbols were not shown filled even though they were using the correct family which did have the triangle filled. After a little bit of trouble shooting I traced it to the surface pattern.
I guess it makes sense because it is a pattern and is a surface, but since this is a symbol and not really part of the Structural Framing family I would expect that this Filled Region not be affected in this way by this action. Anyways... if you are finding that your solid filled moment connection symbols are not filled in, you might want to check to see if the surface pattern is toggle off either in the view or by the element.