Very strange.
I used PPB points many times for tickets and for Y->C and also for C->F upgrades and every single time I got what I requested. Last time it was in November 2014.
But it was also done manually by Lufthansa local office, who made me reservation after what I sent the e-mail to PPB customer service, they deducted points and issued ticket.
My opinion was even opposite --> PPB availability is better than M&M SEN, but that is just a guess.