You actually hire a developer to work on that issue and not something else.

Pretty much what this ensures. Just that the "developer" is a LLM agent.

Yeah except that the agent cannot be held accountable if its fix is crap, or if it went off-track mid-fix.

Pretty straight-forward solution: donors or maintainers don't use the service if it isn't good. But the maintainer is "accountable" (to the extent one maintaining a OSS project is) as they're the one actually providing the prompts and doing QA.