Adding Mannequin animation to my current skeleton causing rotation

Hi.

So the problem is after I import my rigged character from blender, everything looks fine until I open the animation which I retargeted from the Unreal tutorial Mannequin and my character is rotated 90 in 2 angles.

I managed to fix this by rotating the mesh then add keys to the bones, but I would like to know the root cause of this problem, if anyone have an idea.

Thanks for you time!

The problem is how your exporting from blender. The newest version of the export tools will cause the mesh to be rotated weird, and rotating it in the editor or importing will screw it up when its retargeted. This video i made should help you export correctly UE4 adding root motion animations from mixamo - YouTube