You can only iterate the enumerables returned from some AFSDK bulk calls once. This applies to many of the calls on AFData, AFListData, and PIPointList like GetRecordedValues.
I reported this as defect to the AF team some time ago and I was informed that this is "as designed" and has been documented as the behavior.
The workaround is to read the enumerable into a List or array (use LINQ's ToList or ToArray) and iterate that multiple times.
Thank you for your response, that is exactly how we worked around it.