Add nodes for variables using the container type Set/Map do not have a return values that states if an element was successfully added. The tool tip states that this should be the case. This return value was also present in previous versions of the engine (available in 4.17.2-3658906)
Available in: 4.17.2-3658906
Result: The Add node does not have a return value (boolean)
Expected the Add node would have a return value (boolean) that states if an element was successfully added.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-59988 in the post.