Reports unsupported specification when supplying it directly

The OpenAPI 3.0.3 API definition I am trying to integrate into a project is served from API server as yaml. That yml file is valid, e.g. it opens and renders without issues when I try it online via https://redocly.github.io/redoc/. I am using internal http tools to fetch it's raw content due to JWT authentication. So, I am trying to render the docs via React component, like this:

import { observer } from 'mobx-react'
import { useViewModelFactory } from 'mobx-react-viewmodel'

import { ApiDocPageViewModel } from './api_doc.page.vm.ts'
import { RedocStandalone } from 'redoc';

export const ApiDocPage = observer(function ApiDocPage() {
  const { spec } = useViewModelFactory(() => new ApiDocPageViewModel())

  return (
    <RedocStandalone spec={spec} />
  )
})

The important part is the <RedocStandalone spec={spec} /> section. I checked the spec value and indeed it is a string that contains the API specification, e.g. starts like this:

openapi: 3.0.3
info:
  title: API
  version: '1.0'
servers:
  - url: '{server}/api/v1'
....

However, this results in:

Error: Unsupported specification
    at detectSpec (http://localhost:4010/node_modules/.vite/deps/redoc.js?v=7192a3a7:21531:13)
    at http://localhost:4010/node_modules/.vite/deps/redoc.js?v=7192a3a7:36716:56
    at Generator.next (<anonymous>)
    at http://localhost:4010/node_modules/.vite/deps/redoc.js?v=7192a3a7:36632:71
    at new Promise (<anonymous>)
    at __awaiter (http://localhost:4010/node_modules/.vite/deps/redoc.js?v=7192a3a7:36614:14)
    at bundleDocument (http://localhost:4010/node_modules/.vite/deps/redoc.js?v=7192a3a7:36714:14)
    at http://localhost:4010/node_modules/.vite/deps/redoc.js?v=7192a3a7:36701:16
    at Generator.next (<anonymous>)
    at http://localhost:4010/node_modules/.vite/deps/redoc.js?v=7192a3a7:36632:71

I kind of traced this error to https://github.com/Redocly/redocly-cli/blob/main/packages/core/src/oas-types.ts#L103, but not sure if it applies here. Seems it indeed can not recognize the API doc/spec flavor. Which is odd given it works correctly on https://redocly.github.io/redoc. I am using "redoc": "^2.1.3" dependency.

Any idea on what may be wrong in my setup?

The report names the RedocStandalone component and the detectSpec and bundleDocument stack frames; start there and compare how the hosted Redoc demo supplies the same YAML. Done means reproducing the failure with redoc 2.1.3 and documenting or correcting handling of the directly supplied specification.