Even in safe mode (well, just to be sure…)?
What does https://your-discourse.com/posts/{id}.json (where {id} is the new post id) return?
Is there a "reply_to_post_number" key value?
I don’t think I can help further on this unfortunately and I never used Python, I’ll let real connaisseurs try to help you 
Maybe pasting your script would give some clue?