Small and medium sized magma cubes do not drop magma cream. i understand a while back the tiniest cubes were Nerf to no longer drop it however one size up the small and the medium and large seem to have a significantly reduced chance of drop now. the mobs themselves are very rare and it makes no sense to Nerf drop rates when spawn rates already make the drops rare.
i killed several medium sized cubes with a sword that had a looting 3 enchantment and still haven't seen a single drop
can't you craft magma cream (this may be work as intended because i think you can craft it)
Do you have ANY mods installed?
Is the Magma cube the only "buggy" mob?