I was playing my Rogue character and we faced some enemies that could reduce max HP with their attacks. I got hit by an attack, the dm said i took 10 damage, i used uncanny Dodge to halve the damage. The DM said my HP max got reduced by 10, I believe the HP reduction was tied to the damage should the HP reduction also be halved?
I can't say this is true for every monster, but the specter reduces Max hit points based on damage taken.
Life Drain.Melee Spell Attack: +4 to hit, reach 5 ft., one creature. Hit: 10 (3d6) necrotic damage. The target must succeed on a DC 10 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the creature finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
If you're hit for 10 and you reduce that damage by half with uncanny Dodge, the damage taken equals 5 so your hit point maximum is reduced by 5. In this case, you're correct. That may not be true in all cases though. However, generally speaking whatever the DM says is correct. You can argue the point but don't let it derail the campaign.
It's my understanding that abilities which are scaled based on the amount of "damage taken" are based on the amount of damage actually taken, after any damage reduction effects are applied, as opposed to "damage dealt". So if you have resistance to the damage, the effect would be based on the damage after the resistance. Conversely, if you have vulnerability to the damage, that extra effect would benefit from the vulnerability.