In version 2 of the h2 database it's not possible anymore to use a prepared statement parameter for the separator of the STRING_AGG function (PostgreSQL mode).
In version 1 you could do the following:

PreparedStatement statement = connection.prepareStatement("select t1.id, string_agg(t2.value, ?) from t1 left join t2 on t2.t1_id = t1.id group by t1.id");
statement.setString(1, ",");

and you will receive a result that will look like this:

| 1 | v1,v2,v3 |
| 2 | v2,v5    |

If you do the same with version 2 of the h2 database, the result looks like this:

| 1 | v1v2v3 |
| 2 | v2v5   |

So somehow the comma gets lost in the prepared statement. If you don't use a prepared statement parameter for the separator but set it fixed to statement (string_agg(t2.value, ',')) you will receive the correct result. So in general the STRING_AGG function in the PostgresSQL mode seems to work.