It looks like this is changed in TD16.00. The following is from the TD 16.00 Release Summary:
Pushing of aggregations and joins from the outer block into the UNION ALL branches, based on cost
and other factors.
Note that the above includes the phrase "based on cost and other factors".
On that point, do you have stats on the selection and join columns in your test. Are the estimated row counts accurate?
If they are not this won't help.