Harvests ends prematurely

Overview:
When harvesting a collection of 400k records , the harvest completes after only several thousands record have been harvested. Investigation of the mod-oai-pmh.instances table shows that not all the records are streamed from the inventory.

Steps to Reproduce:
Start initial harvest

Expected Results:
All records are harvested
Actual Results:
The harvest finishes after only a portion of the records is harvested. The resumptionToken in the last response is <resumptionToken cursor="cursorvalue"></resumptionToken>. Next record Id and request Ids are missing.

Additional Information:
So far I wasn't able to recreate it our oaipmh testing environment using the same data set but the issue manifest itself on multiple production sites. Here is additional information from the harvesting:
Number of entries loaded into mod-oai-pmh.instances varies (it's been as low as 12K but I've gotten 300K too), and I see the same pattern when watching the system in live time, namely:

• Entries start appearing quickly in mod-oai-pmh.instances
• As soon as they stop appearing, no more will
• I'm able to harvest as many entries that appear in the table
• Empty resumption token is returned

We cannot recreate this issue in our test environments.