SSRS Grouping on Join(LookupSet only returns one column

Question

I'm trying to make a report that returns multiple values grouped by ProductId, and it's only returning one value or the other.

I'm having issues with a report that has 6 columns, one that has ProductId, 3 that return X results, and 2 that return Y results. All fields except for the Id, are Join(LookupSet expressions, and are grouped by row on ProductId. (Some Ids have X values, some have Y, and some have X and Y)
The report returns values correctly, with the exception that if the sql sp returns X values for that Id, the report only shows the X ones, and not the Y, and vice versa.
Am I using the expressions or grouping wrong? Here is the code used:

=Join(
  LookupSet(
    Fields!ProductId.Value, 
    Fields!ProductId.Value, 
    Fields!Price.Value, 
    "Dataset1"
  ), 
  vbCrLf
)

The expression does indeed return all the X or Y values (for example, the X values return 5 results, but then does not return the Y ones)