At this line, we extract the thread history: https://github.com/slack-samples/bolt-js-assistant-template/blob/main/app.js#L188
As the conversation title is actually a message, it is being included in the threadHistory array, which makes the LLM confused.
We also update the title each time the user send a new message here: https://github.com/slack-samples/bolt-js-assistant-template/blob/main/app.js#L188
which makes it even worse for the LLM.
I currently solved it by adding threadHistory.shift(); to remove the first message in the history which is the 'title'.