[2019] Seiryu not waking up in ED3

I'm having some trouble attempting to get my Bisdi (ED2) pet. From what I understood I'm supposed to take my inert black stone crystal into ED3 and talk to Seiryu after killing the Ambassador.

Sadly, Seiryu seems to stay dead to me after killing the Ambassador. I've done 2 attempts now:

• One attempt in normal mode (duo)

• Story mode (single, but was still in group mode)

In both cases Seiryu stays dormant after killing the Ambassador and there are no dialog options.

Just before I do another attempt, I'm wondering if I'm missing something:

1) Does unlocking the pet work in story mode?

2) Does it perhaps have to be a pure solo run?

3) Am I doing something else wrong? Or am I running into some bug.