I don't know if it will solve your problem, but you are creating an ChatOpenAI model, who is maybe, not optimized for Mistral Response.

There is a class for Mistral Model Who looks like that :

from langchain_mistralai import ChatMistralAI

ChatMistralAI = ChatMistralAI(model="mistral-nemo",mistral_api_key=_api_key)

Regards