It sounds like you are sending requests to v3 and not v5, I had a similar issue at first. When I set the default accept header to v5, I was still attempting to send requests to v3. The way I got around this was by specifying the api version within the actual request itself as a query parameter and then everything returned as expected.
that being said, I still noticed some discrepancies between the api documentation and what was returned. For example, when requesting a page of channel feed posts, it says there should be a _total object returned, but when going into the actual returned content, no such object existed. Instead, there was a _disabled object.