In case you set your plan "Visual Style as Hidden Line" it should work if your void is underneth some concrete element and falling withing view range.
Nope not working!
And it is a void inside an object not an object behind or bellow an object!
See attached file.
Go the Family of the Member and add Symbolic Lines>Hidden Line [cut] on the voids in Ref.Level.
Now select these lines and in the properties Visibility/Graphics Overrides (edit) check mark the following
Save the family and Load it into the project.
Yoou must be able to see the hidden lines.
Now if your cut plane levels are not falling within range use View>Plan Views> Plan Region to define a new cut plane for your plank.
Hope it solves your problem!
I will elaborate the necessary detail on my initial response. Sorry about that.
In the Family Editor, go to the Family Category and Parameters button, and define the Structural Material Type say to Precast Concrete.
When you do this, a new edit field will be available "Display in Hidden Views" as shown below which you can change to "Edges Hidden by Beam Itself"
I admit that I do get some display issues after doing this. Namely the void extrusions along the beam. I would suggest dealing with these problematic lines using the symbolic lines discussed earlier. Or perhaps someone else can provide the missing link?