I feel like there needed to be a separate treaty between humans and Yashou, and either they initially didn't see a need for it because the Yashou decided to retreat back to their own territory, so it never existed
This is close to my take as well, except part of the treaty does seem to include Yashou.
Episode 13: Ying Chun: "According to the original agreement we can't interfere their internal conflict. Only then they won't bother us in return." Ya Qing: "Following the old rules there won't be opportunities for Yashou revivification."
So my interpretation is that there's a general non-interference clause in the treaty, but no other stipulations (such as the Envoy's position, or the fact that Haixingren are required to hand Dixingren criminals over to Dixing), because Fu You's intention to retreat meant they didn't foresee a need for that. (And perhaps also because the treaty between Dixing and Haixing was re-negotiated at some point? At any rate, the human government recognises Dixing as a separate polity with separate territory, but it doesn't recognise the Yashou in the same way.)
no subject
This is close to my take as well, except part of the treaty does seem to include Yashou.
Episode 13:
Ying Chun: "According to the original agreement we can't interfere their internal conflict. Only then they won't bother us in return."
Ya Qing: "Following the old rules there won't be opportunities for Yashou revivification."
So my interpretation is that there's a general non-interference clause in the treaty, but no other stipulations (such as the Envoy's position, or the fact that Haixingren are required to hand Dixingren criminals over to Dixing), because Fu You's intention to retreat meant they didn't foresee a need for that. (And perhaps also because the treaty between Dixing and Haixing was re-negotiated at some point? At any rate, the human government recognises Dixing as a separate polity with separate territory, but it doesn't recognise the Yashou in the same way.)