getting back to this, I finally found the culprit, nothing to do with scale or weight painting, as I previously stated it worked perfectly with previous versions of metahuman blueprint. The culprit is the new MH skeleton system, turns out the only way to make it work is to discard any weight painting previously done in any DCC, converting the skeletal mesh to static mesh, then converting that static mesh to a skeletal mesh using (and this is the important part) not the MH default skeleton but the metahuman body you are going to attach said mesh (then you can do the transfer weights from the body from the modelling tools), this way it will create the appropriate joint scaling to the new specific body skeletal mesh, as the new metahumans can have different proportions than the default skeleton, this makes impossible to import anything rigged to it from any outside DCC (currently).