So I have a PO of one of my teams who seems to have delegated their responsibilities to a developer on the scrum team. So when I joined this team as a scrum master a PO was selected and the developers of the team were also selected. By the time that I joined the team they were already on Sprint 2. It was a mess because Sprint 2 had no end date and was not time boxed. Also there were not doing scrum ceremonies and functioned more like a Kanban team. This did not work well, and they were behind 1 month on product delivery. As a scrum master I came in and an did the needful. I educated the team on agile and scrum then eventually we began to scrum and incorporate all the ceremonies properly. Everyone understood their roles and responsibilities and strived to always improve. Until it happened... PO went on vacation and delegated their role to a developer on the team which was fine because the developer is very knowledgeable in the product and was the PO's right-hand man before I arrived. The issue is when the PO returned from vacation, the PO never took their role back. All the decisions that the PO needs to make are still coming from the developer. Meanwhile the PO seldomly joins the team scrum ceremonies. I attempted to have a conversation with the PO however I was brushed off and told that they are busy with other things. I attempted to find out what those "other things" were so I can help where need and again I was brushed off. It has now been 3 months since this has been going on. The other on the team are confused because they either think there are two PO’s or they think the developer is now the PO. In my organization there are some decisions that go in a decision log in the team confluence page, and I have some folks at the program level of my train asking me why the developer is the one signing of on certain decisions. Please note I did escalate this issue a while back when it was fresh and even reach out for advice from other senior scrum masters to no avail. It seems like everyone is afraid of the PO because she knows people higher up in the chain. Honestly I am baffled by this.
If and when value fails to be delivered, who will be held accountable?
The Scrum Guide states this in the section where it defines the Product Owner responsibility.
The Product Owner may do the above work or may delegate the responsibility to others. Regardless, the Product Owner remains accountable.
So, as @Ian point out, the accountability still remains on the Product Owner. However, I do want to point out that the Scrum Guide does not define job descriptions or job titles. It provides responsibilities (roles in previous versions) that need to be fulfilled in order for a team to be successful. So, if someone with a job title of Software Developer can fulfill the responsibility defined in the Scrum Guide for the Product Owner, I see no problem with this. If you have "educated the team on agile and scrum" then you should have included that in the education.
However, the answer above is entirely focused on Scrum as the Scrum Guide defines it. This statement you made makes me think that you may not be in that situation.
... I have some folks at the program level of my train ...
That indicates that you might be working in an organization that is using, or attempting to use, Scaled Agile Framework (SAFe). SAFe does not use Scrum as it is defined in the Scrum Guide. It uses a variation of Scrum that they call ScrumXP. While many terms are identical in both, the meanings are not. While people on this forum can provide you good advice on how to be a servant-leader and potentially navigate the politics and situation you are in, you would probably get a better response if you posted this in a forum that is used by the SAFe community. Putting on my "SAFe hat of knowledge" I would suggest that you discuss this with your Release Train Engineer(or Manager is sometimes used) or to someone in the Program Management level.
@Ian the Product Owner is responsible and will be held accountable for value delivery.