My technical team identified that the issue is that you are using the same mailing (with the same subject line) for each mailing in the series. The app is getting confused by seemingly the same mailing from every step having been sent with the first trigger -- since the other mailings haven't sent yet, its assuming all the mailings are the same, and the identical response metrics reflect this.
Our best practices for automation is to have separate mailings for every step, as this feature was intended for unique mailings during each part of the series. You can easily delete that workflow and setup a new one with unique mailings, and that should resolve your issues here.
Please let me know if you have any follow-up questions on this!