Why does the parallel resource menu show up in Nicene and Post-Nicene Father, First Series Volume 2?
It lists volumes 2, 4, & 5 as parallel:

If I select one of the other volumes, it takes me to the start of the resource, and the parallel resources menu now contains "no parallel resources".

If I open volume 4 or 5 directly from the library, the parallel resources menu is not even shown.
So the problem is specific to NPNF Series 1 v2 (the volume with Augustine's City of God and On Christian Doctrine).
(After reading Bradley's post here: http://community.logos.com/forums/p/24834/184674.aspx#184674, I think this likely stems from the new way of doing parallel resources.)