Although the Note for this call says "Prior to PHP 8.0.0, this function was used to close the resource", I found that PHP 7.4.33 on CentOS is not closing the connection on curl_close.
The workaround if you want to make sure the connection closes immediately after the request is to set the curl option to forbid reuse:
curl_setopt($curl, CURLOPT_FORBID_REUSE, TRUE);