Hey guys, I encounter the same problem, and after tons of research, I may found the problem.
I think the problem is metahuman body roughness map is composite from normal map, you can find this option in that green map.
So I try to use “Modeling Tool Editor Mode” plugin from the previous reply suggested, but it didn’t work for me. In the end, I found this tutorial to make my own baker machine to successfully bake out the roughness map and export as .EXR format(didn’t know why can’t export into .TGA or .PNG), anyway take the .EXR format file into Photoshop then reExport as .PNG file format and ready to go.
But I found some little issue that is after reExport as .PNG file, it seems to be compressed, and the roughness map color becomes a little bit darker~
Hope it’s useful for you! Sorry for the bad English.