org.mariadb.jdbc.message.server.OkPacket#getAffectedRows
It's not used anywhere

MySQL/MariaDB protocol doesn't permit to know that. Only the first insert id value is known.
There is 2 different way to have all insert id's:

Knowing @@auto_increment_increment :

Statement stmt = sharedConn.createStatement();

    stmt.execute("DROP TABLE IF EXISTS t1");

    stmt.execute("CREATE TABLE t1(t1 int not null primary key auto_increment, t2 DATE NOT NULL)");

    try (ResultSet rsIncrement = stmt.executeQuery("SELECT @@auto_increment_increment");){

      rsIncrement.next();

      int autoIncrementIncrement = rsIncrement.getInt(1);

      int rowCount = stmt.executeUpdate("INSERT INTO t1(t2) VALUES ('2006-04-01'), ('2019-04-11'), ('2020-04-11')", Statement.RETURN_GENERATED_KEYS);

      try (ResultSet rs = stmt.getGeneratedKeys()) {

        if (rs.next()) {

          int firstInsertId = rs.getInt(1);

          System.out.println("firstInsertId = "+ firstInsertId);

          for (int i = 1; i < rowCount; i++) {

            System.out.println("Other insertId = " + (firstInsertId + autoIncrementIncrement * i));

          }

        }

      }

    }

of more simply since MariaDB 10.5.0, using RETURNING value :

    try (ResultSet rs = stmt.executeQuery("INSERT INTO t1(t2) VALUES ('2006-04-01'), ('2019-04-11'), ('2020-04-11') RETURNING t1")) {

      while (rs.next()) {

        System.out.println("insertId = " + rs.getInt(1));

      }

    }

the same sql but have two result betwen 2.7.9 and 3.2.0
version-2.7.9 getGeneratedKeys have two ret

version-3.2.0 getGeneratedKeys only one ret

@Diego Dupin

This is still an error in 2.x to return multiple id's in that case. Mysql connector will return one value as well.
For multi-value insert only the first id will be returned.

But MyBatis requires multiple IDs to be returned in <org.apache.ibatis.executor.keygen.Jdbc3KeyGenerator#processBatch>.It was normal to use this in 2.x, but now there is an exception when upgrading to 3.x

Since breaking MyBatis, a solution can be done to add a compatibility option to have the same behavior than 2.x

new option `returnMultiValuesGeneratedIds` added for connector 2.x compatibility. When enable, getGeneratedKeys() will return all ids of multi-value inserts.