Found it. Only stable is broken. proposal-optional-chaining is not getting applied there.
In tests-passed the ?. change includes the proposal-optional-chaining change: DEV: Enable optional chaining in all contexts (#13180) · discourse/discourse@855e854 · GitHub
and in stable only the code change with the ?. was made but the transpiler options were not adjusted: UX: Add auto focus to hamburger and user menu dropdowns (#13165) · discourse/discourse@45dca79 · GitHub