2.5.8 Migration Guide
The engine privatized an array that I needed access too in order to implement the inversed skeletal mesh physics bug fix. I reported this to Epic and got a hopeful promise that they will just fix the actual bug in engine finally in 5.8.1.
However this means that at least initially in the release version of 5.8 that inversed skeletal meshes will have broken physics again (ie: inversed hands).