If the user creates an array parameter on a custom event node by connecting a pin to it, a persistent node will appear on the node that cannot be cleared.
This only occurs if the param is created by connecting a pin to it. If the user creates it from the event's details, it does not occur.
A note appears on the custom event node that will not be cleared.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-99417 in the post.