If you want to let the pelvis move down when crouching and get a more realistic look, you can in the skeleton asset retarget the pelvis to animation, this will correct the flying crouching character ( I did find that the hard way) but in that case, all your root motion animations will get the boot!!!
I mean the capsule component will not move with your root motion animations!!!
I hope that this is fixed in later releases of Unreal Engine