So, my guess is you import straight to Unreal? If so, then your root bone doesn’t exist and the engine attempts to fix this to use your pelvis bone instead.
You will have to add the root bone first. There’s a blender plugin to convert them.
Also, if you are doing walk start and walk end, the setup isn’t the same as simply plugging the speed variable. It’s more complicated with a lot of calculation, that’s what root motion is all about, to use the skeletal mesh to drive the movement component and not the other way around.
I might be mistaken in your animations, but you should use walk loop animation instead of walk start.