Grooms can get improper motion blur and artifacts when rendered with motion blur (vectors and or transforms) and Stencil Layers enabled and in some instances the groom disappears.
This issue is repeatable with the Meerkat Demo.
There is a problem with rendering Grooms bound and attached to animated skeletal meshes when the mesh belongs to a stencil layer in both the Deferred Renderer or the Path Tracer.
The groom does not render properly with lagging motion and low density. In some cases, it appears that the groom disappears completely.
This issue can be recreated in the Meerkat Demo with the Meerkat following the steps to reproduce.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-175577 in the post.