Following up on How does Max-HP reduction affect wild-shaped/polymorphed creatures?, which states:
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 reduction would work any differently. Because Wild Shape/Polymorph gives you a new pool of HP, only that pool is affected by the reduction.
A druid gets seduced by a succubus. They kiss while the druid is in bear form – this is not hypothetical as yesterday exactly this had happened. The druid gets lowered Maximum Hit Points because of this creature effect. So according to the linked Q&A, the reduction would only apply to the bear form.
If the druid reverts back to normal, the HP reduction is not active anymore. What if the druid wild shapes another time, back into a bear: does it get a fresh "pool of HP", or does the Reduced Max HP stay with its bear form until it gets "cured"?
In other words: are shapeshifters actually really resilient against abilities that reduce maximum hit points?
In case it helps to clarify, let's use these numbers:
- Druid: 45 HP
- Wild Shapes into Brown Bear: 34 HP, but reduced to 10 Maximum HP (after two kisses).
- Druid reverts back to normal: 45 HP
- Wild Shapes back into Brown Bear: 34 HP, or still at 10 HP?