NMS-5219, a fix was introduced into the XML Collector to ensure that the collection-set is going to contain only one instance for the node-level resource.
Unfortunately, the code is keeping a reference to this object (i.e. the node-level resource) inside the CollectionSet Handler which is not 100%. This resource must be recreated on each collection attempt (or when a new collectionSet is created).
The problem appeared as part of the troubleshooting for
NMS-6895. But, as this is a new problem, I've decided to create a new issue.