I am updating on the status and solution of this thread for other community users:
On the support ticket logged after this conversation, we clarified that the GET /v3/items/{itemId}/reviews endpoint returns empty response because the itemId has a Review Hub-based review, and not an Item-based review. These look similar but have a different header on the UI.
Review Hub (NOT supported by the endpoint):

Item-based Review (supported by the endpoint):

Currently, no REST API endpoints for Review Hub are officially implemented / supported
- GET /v3/items/{itemId} (with {itemId} being the review ID in the ReviewHub) helps to get basic information about a review in ReviewHub, further endpoints and functionalities are on the roadmap and *may* appear in future versions.