How to get past the 5000 row limit in a SQL Server Reporting Services tablix report?
This turned out to have nothing to do with the Microsoft side at all. The data source we are using is a linked server to Proficy Historian and, by default, only pulls 5000 rows. You have to explicitly tell it either how many rows you want to pull, or tell it RowCount = 0 for unlimited rows. Sorry for the noise!

How To Set Rowcount in Historian OLE DB SQL Query