The curve refactor did not add the curve debugger tab to the skeleton editor, which means that morph targets cannot be previewed there. It would be convenient for some users to still be able to do this, so we should re-add it. Patch is in the linked UDN.
Open skeleton editor
Look for slider in curves tab
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-193662 in the post.