This is kinda hard to explain, maybe I can make sense of it. So I had to add an column to the report which meant modifying one of the four data sets for the report. I was getting data and it was correct in the Tablix. But if I changed the date range then the data for this column would change to what the previous record was.
I have attached two pictures. The correct value and the incorrect value. The group level (comes from a different data set) is correct.
If I run the query for the data set. The column value is correct no matter what date range I put in.