Animation Blend Per Bool Issue - Weird Pop or Jump or something

I created a Handgun Pose / Animation in Maya using ART and exported it to 4.8 using ART. My export presets are set to 2014 FBX with all the boxes ticked as per the Unreal Docs.

Imported the .FBX into unreal and wired up a quick prototype using tab & a flip flop to toggle a bool value inside the AnimBlueprint.

The bool is hen read by a BlendPerBool node which feeds the normal ThirdPerson Animation state or the Handgun pose based on the bool value.

When I hit tab I get a transition but also this weird movement on the feet. I have a feeling this has something todo with the IK Bones attached to my mesh. Any ideas?

You can see the actual happenings here: Animation Pop Example