I notice other Soul of Cinder battle video in youtube, he goes sorcery but when I was fighting Soul of Cinder, in his first phase, he didn't go sorcery at all. Why is that? Is it a glitch?

I remember him only using sorcery after a little while. I may be misremembering. You may also have just gotten lucky.

Might have been a glitch or you might have been lucky, but generally, as time goes on, he should switch between 4 modes with different weapons and spells, sword, sorcery, pyromancy and miracles to sort of resemble the main playstyles of players. It is hard to tell why he didn't do that in your fight.

Is it a definite that he will switch to sorcery for phase 1 when his HP go under certain point or if I end his phase 1 too soon, then he won't goes sorcery? Or it just random?

This isn't the first time this happen. Similar thing happen with my fight with High Lord Wolnir where he didn't summon his skeletons and use swords.

There's an element of RNG to the phases he switches to and you could've possibly rushed him to the second phase to skip a sorcery phase. He does it, for sure.

I have no idea what happened during this fight. I didn't expect the fight to be *right there* so I summoned someone in for fun and that person just tore the boss apart.

It's a bit randon with his phases, particularly the sorcery one, which he doesn't always pull out.