In the test case, pstmt.closeOnCompletion() is called, which should set the PreparedStatement to close automatically when all its associated result sets are closed. The expectation, based on standard JDBC behavior, is that once the statement is closed (either explicitly or automatically), no further operations should be allowed on it.
However, the MariaDB Connector's behavior deviates from this expectation, as it allows a subsequent execute() call on the closed statement, whereas the MySQL Connector J throws a java.sql.SQLException stating "No operations allowed after statement closed."

@Test public void test() throws Exception {

    Connection con = DriverManager.getConnection("jdbc:mariadb://localhost:3366/test1551?user=user&password=password");

    PreparedStatement pstmt = con.prepareStatement("SELECT 1;");

    pstmt.execute();

    pstmt.closeOnCompletion();

    System.out.println(pstmt.getMoreResults(Statement.CLOSE_CURRENT_RESULT));

    try {

        // mysql: java.sql.SQLException: No operations allowed after statement closed.

        // mariadb: true

        System.out.println(pstmt.execute());

    } catch (Exception e) {

        System.out.println(e);

    }

}