Sequencer fails to play anim sequences on linked anim instances when an Additve/Layered FKControlRig track is active

Animations can't be played with Sequencer via slot nodes in linked anim graphs when an additive/layered FK Control Rig is active.  This is because adding an FK Control Rig changes the anim instance that lives on the mesh to be a new UControlRigLayerInstance.  This breaks the behaviour of the 'Use Main Instance Montage Evaluation Data' flag.  Setting that flag on an anim blueprint results in the montage eval data from the anim instance on the skeletal mesh being used, rather than the anim blueprints own eval data.  But because we've now added the UControlRigLayerInstance on the mesh - but we still call Montage_Play on the top level anim instance that the user authored - the montage eval data on the skeletal mesh's anim instance is empty.

This is effectively doing the same thing as nesting the anim instances:

• Control Rig Layer Instance
  • User Main Anim Instance
    • User Linked Anim Instance

We call Montage_Play on User Main Anim Instance but User Linked Anim Instance will attempt to retrieve the eval data from Control Rig Layer Instance

1. Create an animation blueprint which contains a Slot node using DefaultSlot
   1. From Class Defaults, set Use Main Instance Montage Evaluation Data
2. Create a second animation blueprint
   1. Reference the first via a Linked Anim Graph node
3. Drop the second anim blueprint into the level viewport to create an actor
4. Create a level sequence and add the actor as a track
5. Add the level sequence to the level viewport
   1. Set the level sequence actor to Auto Play
6. Set the level sequence 
7. Add an animation track and select an animation to play
8. Add a FKControlRig track
   1. Set the track to be layered/additive
9. PIE (behaviour is only intermittently broken outside of PIE)
   1. Expected behaviour: Animation should play via the slot node in the linked anim blueprint as it would a standard anim blueprint
   2. Actual behaviour: Mesh doesn't animate