An HTTP 304 response is supposed to arrive without a response body. It tells the client that the representation it already has is still valid, so sending the same content again would be unnecessary. If your debugging tool shows an empty body beside 304, that alone does not mean the server forgot to render the page.
The MDN reference explains the conditional request behind it. A client can send a validator such as If-None-Match, and the server can confirm that the cached representation remains usable. That is a different conversation from an ordinary request for a fresh body.
When investigating confusing caching behaviour, I would look at both sides: the request’s conditional headers, the response headers and the cached copy the client is using. Testing with a cache disabled can help isolate the issue, but it changes the conditions. It does not prove that the normal caching path behaves correctly.
The useful question is whether the user sees the right representation under the intended caching rules. Returning 200 for everything might make a network panel look simpler while throwing away perfectly good caching. Understand the exchange first, then change the part that is actually giving the wrong answer.