Using an Avatar Mask for component data

Ahoy! I have some animated component data on my character, and I'm using an avatar mask so that my layered animations don't interfere with each other, but the mask only seems to affect transform data. Is this intended, or a bug in the system?

Currently dealing with it manually by not using Write Defaults and only explicitly animating the data for the parts I need for the layer, but that makes those anims only usable when they are layered in this specific way.

Cheers,
-Evan