Game Version: 1.21.1
Mod Version: ApothicAttributes-1.21.1-2.6.2
Neoforge Version: 21.1.125
Modpack: FTB Evolution 1.11.1

Items given extra attributes with the slot "any" does not work in curios slots.

Command used on item: /apoth add_bonus_modifier apothic_attributes:crit_chance ADD_VALUE 5 any

Here is the item being held and working:

Here is a gif of it being equipped, but not providing any values.

Apotheosis does not have Curios support.

The /apoth modifier command embeds a vanilla attribute modifier in item NBT; any is a vanilla EquipmentSlotGroup, composed of all vanilla equipment slots.

As such, it does not cover curios slots. This isn't a bug.

It might be trivial for me to add a second command to embed modifiers against curios slots, but it can't really be baked into this one.

Well, after a 3-day-long rabbit hole of terrible™ ideas, technically this is now possible. It requires another mod to create and register an EntityEquipmentSlot and an accompanying EntitySlotGroup for each curio slot you want supported through the command (ex: here).

Not sure if I will be creating those objects natively or not. I need to work out the API some more.