Why is my website running GET instead of POST

When Use Javascript to try to POST something using method: 'POST' it for some reason resorts to GET. It gives me the error Cannot GET /openai/generateimage but I don’t understand why it won’t try to POST it.

const response = await fetch('/openai/generateimage', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    prompt,
    size,
  }),
});