It seems that the MultiWayBlend node isn't applying additives correctly. In the attached project are two anim bps, one which dynamically creates an additive and one which uses an additive sequence. Both are blended with additive identity using the MultiWayBlend (seems to be the intended base pose to blend onto with this node) and then applied using the ApplyAdditive node. In both cases, the results of the MultiWayBlend, the resulting output pose doesn't match the pose from which the additive was generated.
My guess is that this is due to the use of
FAnimationRuntime::BlendPosesTogether to blend the additive poses. Disabling the ispc codepath doesn't change the behaviour.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-185035 in the post.