In this page it is stated that you need to rotate your mesh by -90 degrees.
Three things here:
First: Unreal is X Forward
Second: Unreal’s Default Mannequin is Y Forward.
Third: An X forward model doesnt need to be rotated.
Just because Epic itself mixed it up doesnt mean a developer should correct this.
Some users get confused and think it’s mandatory to rotate the mesh. This can cause serious problems with animations because the 90 degrees rotation needs to be taken into consideration