Something that I noticed that may shed some light on the mechanics of the form change:
In the official Japanese trailer, Zygarde 10% takes 120 damage, leaving it with 5/125 HP. When it transforms into Perfect Form, it has 167/287 HP. So it had its max HP -120.
In the low quality one, Zygarde 10% takes 102 damage, leaving it with 23/125 HP. When it transforms into Perfect Form, it has 185/287 HP. So it had its max HP -102.
So when it transforms to Perfect Form, it increases its HP drastically, but retains the amount of damage it already took.