setting up NS record

[paragasu]

let say i have domain example.com and i did setup
ns1.anotherdomain.com and ns2.anotherdomain.com as my name server.

on the domain root server. i add ns1.anotherdomain.com and
ns2.anotherdomain.com,

on mara configuration i add

example.com.  67.167.27.34
example.com.  ns1.anotherdomain.com
example.com.  ns2.anotherdomain.com
ns1.anotherdomain.com    67.75.144.103
ns2.anotherdomain.com    67.75.144.104

it works fine. except .. it return different ns record
if i do $ns example.com
the data returned is
synth-ip-4649a790.example.com and
synth-ip-4649a790.example.com
but sometime it return ns1.anotherdomain.com and ns2.anotherdomain.com
but synth mostly come up (9 out of 10)

i suspect my configuration problem. how to config to make it return the
ns record ns1.anotherdomain.com?