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.
No, but you still keep that bonus for Wild Shape.
Each class has a section called Features, that lists all its class features. These include, for a Barbarian, Rage, Unarmored Defense, Reckless Attack, Danger Sense, etc. The Proficiency Bonus is not listed there.
Chapter 7 of the PHB describes it as
Proficiency Bonus: Characters have a proficiency bonus determined by
level, as detailed in chapter 1.
The character advancement table in PHB, pg16, shows your Proficiency Bonus regardless of class.
When your character gains a level, his or her class often grants additional features, as detailed in the class description. In addition, every character’s proficiency bonus increases at certain levels
The Character Advancement table summarizes the XP you need to advance in levels from level 1 through level 20, and the proficiency bonus for a character of that level.
The Multi-Class chapter also lists Proficiency Bonus based on character level, not class levels.
Your proficiency bonus is always based on your total
character level, as shown in the Character Advancement
table in chapter 1, not your level in a particular class.
It also does not list Proficiency Bonus under special rules for Class Features gained, as Marq points out.
Just for completeness's sake, monsters have Prof Bonuses too, although based on their CR and not Character Level. It is shown in the Monster Manual, pg.8, and on the DMG, pg. 274 (thanks to Slagmoth for the tip).
Keeping the proficiency bonus for Wild Shape
Despite not being a Class Feature, it still carries over on Wild Shape or Shapechange, as confirmed by lead rules designer Jeremy Crawford on Twitter. The ability/spell mentions you take your original proficiencies, and that includes this Proficiency Bonus.
While you're under the effect of the shapechange spell, you use your proficiencies, including your proficiency bonus, except when the stat block of the new form has a modifier (proficiency bonus + other modifiers) that's higher for a proficiency you have. #DnD
@JeremyECrawford
(This also means the Proficiency Bonus doesn't have to be considered a class feature to be kept in Wild Shape.)
Best Answer
The Strength reduction would likely only affect your beast form
Jeremy Crawford has said this regarding maximum hit point reduction (borrowed from this answer to a question about maximum hit point reduction):
Although they are different effects (strength reduction and maximum hit points reduction), the reasoning for one would apply to the other, given that they are both reductions that end on a long rest. However, I cannot find anything published in their Sage Advice publication to confirm JC's ruling.
Taking from the description of Wild Shape (PHB, pg. 67):
And:
That part only discusses hit points reverting back to what you had before, and doesn't explicitly mention your maximum hit points or ability scores returning back to normal, it is only implies by the "While you are transformed" part at the start of this bullet list. Without anything explicit to go on, applying the same reasoning as JC seems consistent with current hit points, at least.