This behaviour is only observed when implementing the function as a custom event from a blueprint interface. This behaviour is not observed in actors where the blueprint interface function is implemented as a function rather than an event, e.g if the function is const.
1. Create a new Blueprint Interface and add a new non-const function to the interface.
2. Add this new Blueprint Interface to an actor and implement this function within that actor. This will create an event node declaring this implementation.
3. Deprecate the function within the Blueprint Interface and compile the blueprint.
4. Observe how within the actor, the interface function implementation has been removed and replaced with a default custom event node.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-198499 in the post.