> The opposite extreme is you have someone on your team who is only able to resolve conflicts by having their boss intervene.

What's wrong with that? Resolving conflicts is the boss' job. So long as the team mate is doing their actual job appropriately, that's all that matters.

> E.g. you leave some critical feedback in a PR review. The author of the PR doesn't like your comments, so they tell your mutual boss, then your boss comes to you to ask why you left the comments in the PR, instead of the author coming to you directly.

The author should not be coming to you directly, going through the boss is the appropriate route. If the author's complaints were unreasonable, it should be the boss telling them that, not you. If your boss is coming to you, it means they feel the author's complaints are at least partially valid, and you should be hearing that from your boss, not the author.

It's not necessarily a bad thing if people bypass the manager to settle things directly, so long as both parties are comfortable with that, but it's not a happy medium.

Handling negative feedback on a PR is a necessary skill for a developer. A manager should only get involved if it becomes hostile. Negative is very far from hostile.