I'm not entirely clear on the relationship between "Sprint", "release", "team backlog" and "program backlog" in your case, so I'll need to keep my answer more general, but it should still be helpful.
If the developer finds the "bug" to be expected behavior but the Product Manager wants different behavior, that doesn't necessarily mean that the expected behavior is expected or desired by the end user. What the developer considers to be "expected behavior" could very well be unacceptable behavior to an end user. In this case, the work to correct the issue may need to happen in the current iteration or release.
This shows a bigger problem. Why is there a delay between finding the bug and talking to the Developers and the Product Manager? Getting everyone together earlier can reduce steps in deciding if the issue found is a bug or a misunderstanding of expected behavior, and if it is a bug, figuring out the impact and priority for fixing it. If it needs to be fixed in the current iteration or release, that may require removing other work, so getting the right people together to have that discussion earlier can reduce the steps in your workflow.
I find the most effective question to ask is what the risk of shipping with the current behavior is. Ideally, all found defects would be fixed before they are released. However, we don't live in an ideal world and need to balance getting feedback on a change or set of changes with introducing potential defects. Only the people closest to the work and the stakeholders can carry out this risk assessment, and the people on the team - the developers, the testers, the product managers - are the right people to do it. The risk assessment can tell you where it goes in priority of all of the other work - the top of the current iteration, closer to the back of the current iteration, or a future iteration.