Changing the world setting (or possibly anything that dirties a map) of a World Partition map that has been checked out by another user and then adding a new actor will cause the Save prompt to not include the added actor nor show up in the source control window.
Possible workaround: opening a different map will correctly list the added actor at the prompt.
The expectation would be that the newly added actor would be listed in the prompt. If you were to add a new actor and then modify the world setting then the actor would be correctly listed.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-149705 in the post.