Merge Join SSIS Throwing Error Despite Sort and Matching Meta
There is a setting where you can tell the Merge Join that the incoming information is sorted, although in the documentation, the Sort task is supposed to properly set these properties.  I wonder if it's because you have other tasks after the sort, and the IsSorted property is getting set back to False.


  There are two important sort properties that must be set for the source or upstream transformation that supplies data to the Merge and Merge Join transformations:
  
  
  The IsSorted property of the output that indicates whether the data has been sorted. This property must be set to True.
  
  
  
    Important
    
    Setting the value of the IsSorted property to True does not sort the data. This property only provides a hint to downstream components that the data has been previously sorted.
  
  
  
  The SortKeyPosition property of output columns that indicates whether a column is sorted, the column's sort order, and the sequence in which multiple columns are sorted. This property must be set for each column of sorted data.
  
  
  If you use a Sort transformation to sort the data, the Sort transformation sets both of these properties as required by the Merge or Merge Join transformation. That is, the Sort transformation sets the IsSorted property of its output to True, and sets the SortKeyPosition properties of its output columns.
  
  However, if you do not use a Sort transformation to sort the data, you must set these sort properties manually on the source or the upstream transformation.


The documentation tells you how to set those settings manually.