Problem stopping an specific instance using upstart

Created the file /etc/init/test.conf with the following contents:
(The behavior is the same if I use $A instead of ${A})

-------- Start of test.conf -----

start on (start_foo A=*)

stop on (stop_foo A=*)

instance ${A}

script
        logger -i -t test "Starting A: $A"
        exec yes
end script

-------- End of test.conf -----

Then when I start two instances, each one with a different instance variable "A"
and then stop one using a specific KEY, it stops both instances. I would have expected only the one with the specified key that matches a specific instance would be stopped:

# initctl emit start_foo A=hello                                                                                                   
# initctl list | grep test                                                                                                         
test (hello) start/running, process 17432
# initctl emit start_foo A=goodby                                                                                                  
# initctl list | grep test                                                                                                         
test (hello) start/running, process 17432
test (goodby) start/running, process 17438
# initctl emit stop_foo A=goodby                                                                                                   
# initctl list | grep test                                                                                                         
test stop/waiting