Additive Blending of Anim Montages horribly distorting animations.

Hi all,

I have a rigged, non-humanoid character from Maya, and we want to be able to blend character animation with dialog in engine, so that we can change how the character moves without effecting the facial animation on the fly.

When I set up animation Montages, and bring them into the character blue print, and combine them with the Additive Blend node, the animations blend, but the character becomes severely distorted, rig and all.

If anyone can shed some light onto this, it would be a great help. Right now I am either thinking that the Additive blend node is broken, or otherwise not doing what I would expect that node to do, or there is something wrong with the assets coming in from Maya.
I have tried this both in 4.9.2 and 4.10.

thanks a lot.