Hi,

Special animations work by temporary replacing the idle animation as the base animation. If this replacement doesn't have transformations for ALL the bones in the skeleton the additive animations, that represents expressions, will collapse the model to just the offsets they contain.
Please be careful however to maintain neutral (idle) pose for the face bones throughout the special animation, because the tracking is still active for the face, and if you bring some movement, a smile for example, this movement will interfere with the tracked expressions.

So, in short, you should export a special animations with keys for all the bones in hierarchy and maintain idle pose for face bones.