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Engine does not handle in-between blendshapes
Result: In-Between Blendshape is imported as it's own unique Morph Target, but there is not curve imported for it.
Expected: In-Between Blendshapes are either handled as their own Morph Target with their own anim curve including adjustments to the main Morph Target anim curve OR we handle them as part of the main Morph Target itself.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-45648 in the post.