You are 100% sure that those PCs are requiring PoE? If you check with "show power inline" (I assume the switch is a Cisco one) you see power injected on those ports?
I'm asking because you may look at your problem from wrong perspective and not the PoE to influence the UDP packets, but the cards in your PCs.
Did you tried to swap the PCs in those ports to see if also the behavior is swaping?
Let me know if this helps you!