ReportAggregators not including groups/stratifiers/etc. from both Reports

Correct Behavior:

MeasureReport A {
   Group 1 = 8
   Group 2 = 3
}

+

MeasureReport B {
   Group 2 = 8
   Group 3 = 5
}

=

MeasureReport C {
   Group 1 = 8   <- Included from A
   Group 2 = 11  <- Included from A and B
   Group 3 = 5   <- Includes from B
}

Current Behavior:

Current Behavior:
MeasureReport A {
   Group 1 = 8
   Group 2 = 3
}

+

MeasureReport B {
   Group 2 = 8
   Group 3 = 5
}

=

MeasureReport C {
   Group 1 = 8   <- Included from A
   Group 2 = 11  <- Included from A and B
   Group 3 = MISSING! Wasn't present in A so it wasn't added! 
}

Start by locating ReportAggregators and inspect how groups, stratifiers, and related data are copied when combining two MeasureReports. Reproduce the example with Groups 1–3, then verify that the combined report includes entries unique to either input and sums shared entries as described.