By default for all Aux Copy Operation:
- A media which contains oldest backup job is copied first.
In regards to the "Last Full Backup radio button", all that is doing is picking up the last Backup for a specified period (i.e. Weekly, Monthly, Quarterly or Yearly) to be selected for Aux Copied.
It is strange that you are seeing newer Selective Full Backup being Aux Copied first.
When you review the Jobs on the Secondary Copy, and check the Transfer Time, do you see any discrepancy (this should show you when Job has been Aux Copied)