No, its intended design. Before bolstering, it checks if its a PvP or PvE item. So with 2 PvP pieces and 1 PvE piece, it is being counted as PvP gear and receives no bolster. (So you lose expertise for the armoring)
But if all 3 parts were PvE, the entire piece would be flagged as PvE and all 3 slots will receive bolster.
Pretty much this. I suspect their issue resolving this is what led them to just removing expertise from war hero gear altogether.