What happens when Aid ends on a target who has suffered maximum hp loss?

Question

The spell aid temporarily raises a target's hit points and maximum hit points by 5:

Each target's hit point maximum and current hit points increase by 5 for the duration.

A vampire's bite, for instance, causes necrotic damage with a rider that reduces the victim's maximum hit points:

The target's hit point maximum is reduced by an amount equal to the necrotic damage taken

If a character under the effect of aid is bitten by a vampire (or subject to any other effect that reduces their maximum hit points), what happens to their maximum hit point total when aid subsequently expires or is dispelled? I can see two interpretations:

1. The character's maximum hit points drop by 5 again, so ultimately their new maximum hit point total is whatever it would normally be less the maximum hit point reduction they suffered

2. The maximum hit point loss suffered reduces the increase granted by aid first, and maximum hit points already lost this way aren't lost again when aid ends, so the character is up to 5 points better off than they would have been without aid

Essentially, aid might or might not act as a buffer against maximum HP loss after it expires, but I'm not sure which interpretation is best supported by the rules.

Answer 1

Their maximum hit points will decrease twice (from the vampire and from aid ending)

The aid spell states:

Each target's hit point maximum and current hit points increase by 5 for the duration.

It does not state that you gain temporary hit points or that these hit points and this change to your maximum are somehow special and act in any way different from regular hit points. Thus, there is not sense of aid's hit points being last or first, or really anywhere in your health pool.

This is also supported in this answer to the question "What happens to the current hit points when Aid ends?" which argues that your current hit points lower when aid ends, regardless of anything else. Similarly, I would argue that your maximum hit points lower regardless of anything else. The current and maximum hit points granted do not exist solely at the "top" of your hit points pool.

Because the bolstering energy of aid does not explicitly define itself as some special form of hit points (ex: temporary hit points or an abjuration Wizard's ward), they are to be treated just as any other hit points. They do not have any "location" in your health pool, and do not vanish when the "top" of your pool falls away.