I am aware of Jeremy Crawford's ruling that maximum HP reduction applied to a Wild Shape form does not apply to the druid's true form when Wild Shape reverts. However, this doesn't answer the question of what happens when the attack that causes the maximum HP reduction is also the attack that causes the druid to revert.
This is certainly a related question to the appropriate order of damage and HP reduction effects, with the added wrinkle of resolving when the druid reverts. Assuming someone can cite the answer to the linked question (which was made more by reductio ad absurdum than a cited source), I can see four likely scenarios:
The wild shape takes damage and reverts, following the full max HP reduction is applied to the druid. (This is, to me, most logical scenario, and results from the order damage-revert-max HP reduction.)
The wild shape takes damage and absorbs as much of the max HP reduction as possible, applying the remaining damage and max HP reduction to the druid. (This is hard to sell, as the max HP reduction is applied at a time when no rule says it should be, but based on Crawford's tweet above seems to be the rule as intended.)
The wild shape takes damage, and max HP reduction is applied, after that, the form reverts and additional damage is applied to the druid. The druid's max HP is not affected. (This is the most lenient case, but seems more likely to be correct than case 2 if the correct order of application is damage-max HP reduction-revert.)
The druid dies before it reverts because its wild shape form was reduced to 0 maximum hit points. (This is the harshest case and results from the same order, damage-max HP reduction-revert. This seems more likely again than the previous two, as "The target dies if this effect reduces its hit point maximum to 0," is not like Disintegrate's check after the spell's damage has been dealt.)
When the same attack that causes the druid to revert from Wild Shape also reduces their max HP, what happens?