It looks like you are blending animations with diffrent root bone orientation. Check the bone default rotation and if that is the case, you will have to pick one skeleton setup and stick with it.
They are all Kubold’s animations from the marketplace, I’ve been using the exact same animations without a problem on the same skeleton. Even ones that used to work fine now do it if I re-import them. I’m certain it’s not the animations causing it.
I had a similar effect happening too. Mixamo skeleton/animations have a certain root bone orientation, and UE4 mannequin skeleton/animations have another. The root bone rotates during the blend and causes that dip motion. Have you checked the local root bone orientations in the 2 animations and skeleton? As SerzaNT said, it really does look like at least 1 of them doesn’t match the others.