Implemented interfaces are not added to the list of assets to nativize when the blueprint class that implements them is flagged for nativization, saved, and compiled. This results in a package failure if packaging is then attempted.
Found on 4.15 Binary CL-3269047
Result: The implemented interface has not been added to the nativization list
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-41072 in the post.