Tansaction Isolation Level is not applied if set before connect

SQLAllocHandle(SQL_HANDLE_DBC, Env, dbc);

CHECK_DBC_RC(dbc, SQLSetConnectAttr(dbc, SQL_ATTR_TXN_ISOLATION,

    (SQLPOINTER)SQL_TXN_READ_COMMITTED, 0));

 SQLDriverConnect(dbc,...);

  SQLAllocHandle(SQL_HANDLE_STMT, dbc, Stmt);

  /* Checking that after connection we have that isolation level */

  CHECK_DBC_RC(dbc, SQLGetConnectAttr(dbc, SQL_ATTR_TXN_ISOLATION, &isolation,

    SQL_IS_POINTER, NULL));

// Here it's gonna be fine

  is_num(isolation, SQL_TXN_READ_COMMITTED);

  OK_SIMPLE_STMT(Stmt, "SELECT @@transaction_isolation");// or @@tx_isolation

  SQLFetch(Stmt);

 // Here the value won't be really "READ-COMMITTED"

  IS_STR("READ-COMMITTED", my_fetch_str(Stmt1, buffer, 1), sizeof("READ-COMMITTED"));