UDN customer mentioned:
"there looks to be support for motion vectors as exported from DCC apps (for rare cases like fluid sim), but regular in-engine motion vector calculation for regular motion blur seems to be missing. Is that correct? I can't get my blendshapes to motion-blur like they do using regular (non-streaming) alembic import"
So it seems that when the Alembic is imported as a regular GeometryCache asset, there's motion blur on the GeoCache as expected, but the same Alembic loaded as a streaming GeometryCache does not.
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-129591 in the post.