Mesh Space Additive behave differently for non-animated bones and extra bones on a target skeleton

Typically, non-animated bones behave the same way as extra bones on a target skeleton. However, it appears that this isn't the case for mesh space additive sequences.

When a bone is present on the source but not animated, it will still contain motion in the compressed data because that motion will come from its parent bones (we are in mesh space). As such, non-animated bones follow their animated parent.

When a bone isn't present in the source but present on a target skeleton we animate with, the extra bones will be initialized to the additive identity. As such, the bones will not be impacted in mesh space by the additive sequence and those bones will remain where the base sequence placed them in mesh space. As a result, they disconnect from their animated parent bone.

The user suggests initializing missing bones with the parent bone rotation, which seems a reasonable fix.

See here: Mesh-space additive animations don't retarget properly onto meshes with extra bones (unrealengine.com)