In the end, it comes down to this ...
WotC doesn't agree with itself on how polymorph works. The ultimate authority, the PHB, is unclear as there are two perfectly logical, yet conflicitng, ways to read the rules with regards to hps and polymorph. What authority comes next (sage articles, sage advice, DMG errata, etc ...) is unclear. In the end, nothing clear of a set of PHB errata specifying whather hps will change will end this argument decisively.
Either interpretation opens up a lot of problems in a game. You are best served by selecting which set of problems you wish to endure and using the rule associated with that set of problems.
I wrote my own versions of polymorph for my game. My version is more complex, but it doesn't have any of the balance or playability issues present in the two interpretations of the PHB polymorph. If you use polymorphing magics a lot, I suggest you look into doing the same thing.