As demonstrated in this fiddle, array observers cause new records to be materialized before they have a hash of attributes to materialize.
Side Note: If you include the patch from #449, you see a different error: the second new instance of an observed model being created with an id of "undefined" (the literal string; not undefined). This is because materialization winds up using the record's clientId before it even has a clientId. Either way, the root problem seems to be the triggering of materialization unnecessarily.
This will be fixed by the updated #449. See that pull request for a detailed example of the problem.
as #449 is closed, I assume this one may be closed too. @pivotal-medici could you confirm ?
I believe this is now fixed. @pivotal-medici, please let us know if not.