Reimporting an asset with Morph Targets does not refresh the Morph Target Previewer.
Any newly-imported Morph Targets appear to not have been imported.
Closing and reopening the Skeletal Mesh Editor refreshes the panel, populating it with the missing Morphs.
This issue can also be reproduced using the Reimport Mesh / Reimport All Meshes option on the Skeletal Mesh Toolbar.
I can successfully reproduce this issue as far back as the 4.17.2 Binary (CL 3658906); it is not, therefore, labelled a regression.
The Reimport process completes. Morph Target Previewer remains empty
Morph Target Previewer populates with mesh Morph Targets once reimport completes.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-60797 in the post.