[4.19] SetMorphTarget fails if shadows toggled !!

I have an Actor with a single SkeletalMeshComponent. The SkeletalMesh has morph targets. I can use the SetMorphTarget node (and the corresponding C++ function) to easily set the morph values. How ever if I switch shadows on and off (in BP or code) the Morphs fail completely reverting to a zero value. I did a quick test and in the image attached all I did was toggle visiblity and the morphs stop working. The C++ code for the node set the SkeletalMesh as dirty and I assume that this resets the morph values to zero… which shouldnt happen. Any suggestions?