Reset changes in prototype is not applied to the speed tree preview

Steps to reproduce:

See attached gif or:

1. Download attached project
2. Open t1 scene
3. Click on "Broadleaf_Desktop.spm" in Project view
4. Change Main Color in the Inspector/Broadleaf_Desktop Import Settings
5. Click "Apply & Generate Materials" button - notice that changes are applied to the previews (in Project View and in Place Tree Tab)
6. Click on "Broadleaf_Desctop.spm" in Project view
7. Reset the changes in the Inspector/Broadleaf_Desktop Import Settings
8. Click "Regenerate Materials" button

Actual behavior:
Preview is not updated after reset (both in Project View and in Place Tree tab)

Expected behavior:
Preview is updated after reset

Workaround: The only way to see the changes in the preview is to click "Reimport" in the context menu in Project View.

Not a regression - reproducible in 5.5 and in 5.4