Why is this SSRS Report rounding a BIGINT Value to the nearest Thousandth when exporting to Excel?
Like MguerraTorres suggests, SSRS has probably got confused somewhere, and the value suggests it's getting cast to some kind of floating point datatype somewhere along the line. 

Try checking the datatype SSRS is assigning by looking at the xml in the .rdl file raw text.

for a simple bigint column I'm seeing an SSRS field definition something like this:

&lt;Field Name="i"&gt;
  &lt;DataField&gt;i&lt;/DataField&gt;
  &lt;rd:TypeName&gt;System.Int64&lt;/rd:TypeName&gt;
&lt;/Field&gt;


You might then be able to edit the datatype directly in that text stanza and trick SSRS into sorting itself out