Not sure if this will help and also I’m on 4.27 but should be the same, open up the skeletal mesh and under asset details scroll down and under skeletal mesh make sure you have the correct post process bp for your body size.
Well, those are all correct. I have created an Animation Blueprint for metahuman, and in its preview mesh option changed the mesh.
My setup is I have one Parent MetaHuman blueprint, and couple of child blueprints to keep only one animation blueprint for all the different metahumans.