For $29/mo, we provide access to advanced level technicians who will answer your questions on any Macbook board related matter to the best of their knowledge promptly & walk you through how to solve your problem so you can deliver a working board to your customer.
12.8 is too high which is strange. Is your multimeter battery low? Check accuracy of the meter, PPBUS should never be higher than 12.58V.
If meter is OK then check sense resistor circuit for ISL6259.
See if the board does anything in SMC bypass mode. Hold power button and connect AC, hold for 5 sec. more and release and press again. PPBUS should come up at 12.23V now.