Usually it is considered that developers can engage with users and stakeholders in the way you've asked as an exception rather than the rule.
You always have three roles:
- Stakeholder who knows "what to do".
- Developer who knows "how to do".
- PO/BA in between who facilitates transformation of "what to do" into "how to do".
There is always someone in each of those hats. It might be one person who seats behind the screen writing new social network in three hats on top of each other. Or a huge company with departments walking around in one narrowly specialised hat each.
Going back to your question, how to actually make a decision "should developers be talking to users during discovery" or not? It depends solely on the goal you want to achieve, is it good for your goal or not. And I assume that the goal is the vision of organisation as a mechanism behind the product delivering it as a value in the most efficient way possible.
If current hats(roles) distribution across people is the same as it should be in your organisation - then you are doing everything right. But since person with PO title exists we can conclude that this very person is supposed to carry PO hat, and in reality cannot. Hats distribution seems to be wrong and the answer to your question in your situation is: no, they should not. (Or someone has a wrong title not matching real role.)
Also some examples when you want direct engagement not in the way you've asked:
- Let people meet in person to become acquainted. It is more comfortable for us when we know real person behind username on the screen.
- Let development team listen, to share the pain as Majaii mentioned already. We love to help and know about relief of actual person.
- Let all project participants be gathered together and literally sit on one side while you present product/project changes and results.
Development team has nothing to do since PO as a bottleneck can't load the team. We can alleviate the bottleneck bypassing it and load development team from users directly. Should we do that?
Unfortunately this is where actual art of project management takes place. You need to consider all the risks, consequences and benefits both short and long term. Some of them are already mentioned in other answers.
P.S. Your question assumes, that stakeholders are somehow more technical than PO. Moreover, development team needs to obtain technical knowledge from stakeholders. This sounds very suspicious.