But here's the huge discovery thanks to user "Rhys1201" on Discord that noticed it in the gameplay trailer after reading this thread.
The lost cinematic of miquella handing his hand to the scadutree was taken from top the stairs of the divine gate. But the corpses are dark red and match perfectly!

Update:
Firstly, some objects bleed when hit in the boss arena. This is inconsistent as there's no pattern on which objects bleed. This seems to be a parameter left out by the artist by mistake during the rework of the gate.
Thanks to Abraham Link for pointing it on Discord.

I recognized some familiar patterns in the streaks going across the texture. It's the lava texture again.

It appears like a quick tweak made to the color texture to fit its surroundings, and "restoring" it is easy.
This is at best a guess of how it could have looked like originally, but the colors look coherent.