Oh no! Not that stupid dipping problem on blend! That was a nightmare back when I first picked up UE4 trying to use non-mp anims with mp anims.
Something to do with the root rotation I think. If you turn on visualizing the root bone I think you can see it during the blend. At least you used to be able to.
I second this. Hopefully this can be fixed, otherwise you can ONLY use mixamo anims exclusively with mixamo imported meshes/anims as blends will all be wonky like this.