Using the meta specifier MustImplement will cause the associated dropdown to only list classes/blueprints that implement the provided interface. If a blueprint has not been opened/loaded in the editor it does not appear in the dropdown selection
Ex: right click & Create Child Blueprint class
Opening the blueprint or right clicking on it will add it to the dropdown
No, same behavior occurs in 4.16.3
ParentBP will show up in the dropdown list but ChildBP doesn't. Opening ChildBP or right clicking on it will cause it to show in the list
Blueprints with the given interface appear in the dropdown by default.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-51321 in the post.