FWIW, it is "normal" for P class to never be released on most, if not all, 78X flights based on my occasional tracking--not far out, not close in, not even within the SDC window. There was a window of several weeks where P class was available on all 78X flights within a month or two out, but that exceptional opportunity seemed to close last weekend and now we're back to the status quo. If this persists, then the only way to get on the 78X would be to hope for a schedule change, pay for a $1,000+ Z fare, or wait for P class to be made available en masse for a short period of time again.
Bottom line is, if you want to take advantage of this deal, I'd not wait for P class to be made available on the 78X and buy whatever flight works best for you at the price. I'd put my money on this fare going away before P class on the 78X comes back.