I acutally suspect the protective paint covering the middle leg has impurities, maybe some metalic fragment or even dirt mixed from air flow, during the manufacturing process.
It may not has problem during testing phrase in factory, but accumulated too much heat during actual use.
It can be seen from the photo that the paint cover was almost completely gone while not much damage to the chip. There is a small burn mark on the corner of the chip but that is from the fire of the burning paint.
The chip packaging is intact. So the chip did not burn itself.
Related to PSU?
Really doubt. This is motherboard manufacturing defect.
Buy a new mother board?
You do need a replacement. Contact your seller and arrange RMA shipment. You may have to pay for one-way (not both ways) shipment. The whole process can from 2 weeks to 2 months(there is no standard and depends on situation). You have to confirm that with your seller.
If you are in a hurry to build the machine, then yes, you may have to buy an extra board.