Not a biggie, but was replacing a thumb for a TV series I didn't like, so I deleted it and added a new one, except I forgot to specify Thumb so it overwrote the Primary. No prob, I deleted the thumb shaped Primary and told it to Refresh Metadata and ticked Replace Existing Images. It didn't. Tried again. It didn't.
So should I actually expect it too in this situation (adding in a missing Primary which was deliberately deleted).