The same prepared query yield "-1" the first six times and then "-1.0"

Hello,
I am using
postgresql version: 15.3 (Debian 15.3-0+deb12u1)
org.postgresql.postgresql JDBC driver version: 42.6.0
via Java 17.0.7

I discovered an unattended (for me) situation: when I execute
10 times the same prepared query the result is not always the same.

I wrote a little test case to show this.

this is the db that I am using:
 CREATE TABLE number(
   name character varying(30) NOT NULL,
   dim1 real DEFAULT '-1' NOT NULL
 );
 insert into number (name) VALUES('first');

and the test program:
static final String DB_URL = "jdbc:postgresql://192.168.64.7:5432/testdb";
static final String USER = "user";
static final String PASS = "password";
public static void main(String[] args) throws Exception {
 Connection conn = DriverManager.getConnection(DB_URL, USER, PASS);
 for(int i=0; i<10; i++) {
   try( PreparedStatement istruzioneCelle = conn.prepareStatement(
       "SELECT dim1 FROM number WHERE name='first'") ) {
     ResultSet rs = istruzioneCelle.executeQuery();
     rs.next();
     System.out.print("p: "+rs.getString("dim1")+"\n”);
   } catch (SQLException e) {
     e.printStackTrace();
   }
 }
 conn.close();
}

The attended result was a sequence of ten equal values but this is the actual result:
p: -1
p: -1
p: -1
p: -1
p: -1
p: -1.0
p: -1.0
p: -1.0
p: -1.0
p: -1.0

All works fine if I open and close the connection after every single query
but in production I am using pooled connections.
This is what I can read in postgresql logs (it seems that after 4 queries
the statement becomes named and the result changes after the second call to
the named query):

2023-08-21 11:51:50.633 CEST [1511] user@testdb LOG: execute <unnamed>: SET extra_float_digits = 3
2023-08-21 11:51:50.634 CEST [1511] user@testdb LOG: execute <unnamed>: SET application_name = 'PostgreSQL JDBC Driver'
2023-08-21 11:51:50.644 CEST [1511] user@testdb LOG: execute <unnamed>: SELECT dim1 FROM number WHERE name='first'
2023-08-21 11:51:50.648 CEST [1511] user@testdb LOG: execute <unnamed>: SELECT dim1 FROM number WHERE name='first'
2023-08-21 11:51:50.649 CEST [1511] user@testdb LOG: execute <unnamed>: SELECT dim1 FROM number WHERE name='first'
2023-08-21 11:51:50.650 CEST [1511] user@testdb LOG: execute <unnamed>: SELECT dim1 FROM number WHERE name='first'
2023-08-21 11:51:50.651 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name='first'
2023-08-21 11:51:50.651 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name='first'
2023-08-21 11:51:50.653 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name='first'
2023-08-21 11:51:50.653 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name='first'
2023-08-21 11:51:50.654 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name='first'
2023-08-21 11:51:50.656 CEST [1511] user@testdb LOG: execute S_1: SELECT dim1 FROM number WHERE name=‘first'

Can I do something to avoid this problem?

thank you
Edoardo