Test case -
If a function is of the form

() => {
  return <some_jsx />;
}

the prop-types rule is checked as expected.

But if the function is of the form

() => {
  const returnValue = <some_jsx />;
  return returnValue;
}

it seems the the prop-types rule is not checked and no lint errors are thrown if there are no propType validations.

Possible reason - Referring to this comment, I think the plugin identifies a function as a React component if it returns some JSX. In the case above, the plugin somehow fails to identify it as returning some JSX.