I was wondering recently, while looking at the Aasimar race in Volo's Guide, which inherently gets resistance to necrotic and radiant damage, what happens when you get hit by an attack that reduces your maximum hit points on top of dealing (necrotic) damage.
So, for reference, I took a look at the vampire's bite attack (emphasis mine):
[...] The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount. [...]
The descriptions of most other hit-point-maximum-reducing effects feature similar wording (wight, specter, ...). Some effects are different, such as the (demi-)lich's legendary action "energy drain", which involves no mention of necrotic damage.
The description of the Mummy is a bit misleading, however: (emphasis mine)
Rotting Fist. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage plus 10 (3d6) necrotic damage. If the target is a creature, it must succeed on a DC 12 Constitution saving throw or be cursed with mummy rot. The cursed target can't regain hit points, and its hit point maximum decreases by 10 (3d6) for every 24 hours that elapse. If the curse reduces the target's hit point maximum to 0, the target dies, and its body turns to dust. The curse lasts until removed by the remove curse spell or other magic.
Now, while descriptions such as the vampire's bite attack are pretty clear and the max hp reduction would in fact be affected by necrotic resistance, I think the mummy's rotting fist is a different matter.
Of course, as per RAW, there is no connection between the initial necrotic damage and the hit point maximum reduction. However, since it's the same (dice) value, one could assume a connection between those two effects.
So, to be clear, I'm aware that, as per RAW, there is no connection. Should a DM, however, allow this "reduction of the reduction", due to the interpreted coupling between the effects?