When you Wild Shape/Polymorph you "assume the beast’s hit points" thus essentially creating a new, separate pool of HP from your own, original form similar to how Temporary Hit Points work, as Alexis Wilke has stated.

Damage taken in animal form doesn't affect your original form's HP unless you're dropped to 0 HP in animal form and there's excess damage. Nowhere is it suggested that max-HP reduce would work any differently. Because Wild Shape/Polymorph gives you a new pool of HP (as supported by Jeremy Crawford in the link below), only that pool is affected by the reduction.

So, using your example, if a PC has 30 HP in their original form and transforms into a beast that has 50 HP, the PC effectively has 50 HP. If the PC has their max HP reduced by 30 while transformed then they don't die as a result of having 0 HP because they're using the beast's HP and they still have 20 HP left in that pool.

As for whether the max HP reduction carries over to your original form when you revert, according to Jeremy Crawford, lead rules designer, the answer is no:

Jonathan Longstaff
^@pukunui81
@JeremyECrawford What happens when a wildshaped druid that has had its HP max reduced reverts back to normal? Does the reduction carry over?

Jeremy Crawford
^{@JeremyECrawford}
Wild Shape—a reduction to hp maximum doesn't carry over from your beast form to your true form or vice versa.

This exact question was answered just this Friday in the most recent FAQ posting on paizo's site.

Wild armor and other transforming armor: When I use a wild armor and gain the armor’s benefits, what restrictions, if any, apply to me? In general, when I transform with a polymorph effect and some of my gear melds into the form, what restrictions do I have for melding with large amounts of heavy gear? What about other types of transforming armor?

If you were in medium or heavy load from encumbrance before transforming, you continue to take those penalties in your melded form. Otherwise, ignore the weight of melded items and calculate your encumbrance in your polymorphed form entirely based on non-melded items. When wearing melded armor and shields, if you gain no benefit from the melded armor, you still count as wearing an armor of that type, but you do not suffer its armor check penalty, movement speed reduction, or arcane spell failure chance. If you do gain any benefits (as with the wild property), then you do suffer the armor check penalty, movement speed reduction, and arcane spell failure chance. This also applies to all other situations where you or an armor transform: you always count as wearing an armor of that type, and if you gain any benefit at all from the armor (such as mistmail), you apply the armor check penalty, movement speed reduction, and arcane spell failure chance.

It can be found HERE