SharePoint 2010 CAML Query fails when indexed columns used

I have an SPQuery that has been working fine until my list hit 500 items.

q.Query = "" + DefectID + ""; This was successfully returning 1 item where DefectID matched my provided value.

Once the list hits 500 items, the query returns 0 items, no error message, no exception. If I delete 1 item and take the list down to 499 items, it works again.

The list had individual item permissions, but as part of my debugging I have reset the role inheritance of the list and also every item in the list.

I have tried providing a row limit of 2, and 2000, same result

Have tried Query override throttling, same result. Changed the web app throttle limits to way above my lists number of i开发者_运维知识库tems, and change the large query window to 24hrs, same result

The column I am querying has an index on it (I am expecting about 30k+ items per year in this list). If I delete the index the query then works when items in the list are > 500. If I create the index again, the query then returns 0

Thanks