Hi I have created basic character with mutable plugin and turned on “Groom extensions for mutable” plugin as and I got it to sort of look ok in mutable (the collision isn’t working but at least i see the hair).
Try zooming in and out on your character in the level to see if there’s an issue with lods. Also select the character instance in the level and in the details make sure a groom component has been added (should be name GroomComponent_0 and be tagged with Mutable).