Certain creatures have abilities which can reduce a character's maximum HP, and usually if it gets reduced to 0 the character dies outright.
Suppose a HP30 PC is wild-shaped/polymorphed to a creature with 50HP, they get into a fight with a Wraith and take a few hits dealing a total of 30HP. If they failed the con saves, that PC's max-HP is reduced by 30, but it's still at 20.
An interesting, perilous situation.
Do they die instantly? Would feel a bit unfair since they're standing there with a bunch of HP. Is the damage just shrugged off like normal damage upon return? The Druid's wild-shape section is quiet on status conditions, though it's pretty blatant about HP:
When you transform, you assume the beast’s hit points and Hit Dice. When you revert to your normal form, you return to the number of hit points you had before you transformed.
That sounds like a free pass, but it would reduce the danger of these fights considerably. I've been assuming the PC becomes a sort of 'dead man walking' where if they revert the HP reduction will carry and they'll die instantly. But I'm not sure.
If that's the case, they've got a 'Crank' like situation where the PC has less than an hour (before the wild-shape/polymorph wears off) to find a Heal or Remove Curse.
Best Answer
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: