Rest API: wp_verify_nonce() fails despite receiving correct nonce value

First, nonce for an action is the same when the user is not authenticated/logged-in. (because the user ID is always 0)

Secondly, you’re getting false because you didn’t specify the nonce action which is wp_rest in your case:

// Correct usage - a valid action name is set (wp_rest).
wp_verify_nonce( $clientNonce, 'wp_rest' );

// Incorrect usage - empty action; always returns false.
wp_verify_nonce( $clientNonce );

And if you are actually trying to authenticate the request using the standard cookie authentication, then you should use the X-WP-Nonce header to send the nonce and not the request body/payload:

const fetchAllCoupons = async () => {
  try {
    const response = await fetch(`${apiBaseUrl}/pluginFolder/1.0/loadAll`, {
      method: 'post',
      headers: {
        'Content-Type': 'application/json',
        'X-WP-Nonce': clientNonce // <- here, send the nonce via the header
      },
      body: JSON.stringify({
        // WordPress will not use this when verifying the associated user in the request.
        clientNonce,

        // but WordPress will not use this, either. Because we're sending a JSON payload.
        _wpnonce: clientNonce
      })
    });
    ...
  } catch (e) {
    ...
  }
};

Or you can also send the nonce as a query string (in the URL) like so:

`${apiBaseUrl}/pluginFolder/1.0/loadAll?_wpnonce=` + clientNonce