Starbuster core.. is it still existent?

[Glockzawa]

Using Tmod legacy *1.3*
Doing a calamity game and im farming for summoner and i found out about the starbuster core thats good for another upgrade. But i cant get the enemy it drops from to actually spawn, i've beat astrum deus, im post moonlord
it
will
not
spawn??

[Mueril]

have you killed astrum aureus? ya might end up skipping that boss