I think I actually found a schematic for this type of board.
http://elecfreaks.com/store/download/datasheet/shield/NANO_pro_Shield.pdf
According to it, the power goes from the shield to the Nano, then returns back to the shield as VCC (5V), before being downregulated to 3.3V. Is it your understanding too?
I think it would better if the shield converted DC to 3.3V directly, without having to go through the Nano first.
NANO_pro_Shield.pdf (16.5 KB)