I don't know exactly what is causing the problem. Are you connecting Tableau directly to this stored procedure? Tableau would normally execute a stored procedure and insert the data into a temporary table in the data server. It's possible for things to go wrong at this stage. Maybe you should contact Tableau Support.
My suggestion will be to run the stored procedure in your SQL database and populate an actual database table with the SP results. You can then connect Tableau to that table. This will make it very efficient and you can easily go check the physical table when troubleshooting issues of this nature.
Hope this helps.