With client, if the isolation level is set with an SQL Statement such as "SET CURRENT ISOLATION = RS" the value of Connection.getTransactionIsolation() will not be correct.
jdbcapi/checkDataSource has tests for sending SQL statements to change the isolation level:
s.executeUpdate("set current isolation = RS");
printState("SQL to change isolation in local", cs1);
I will disable these tests and reference this bug number.
It is not really clear how to handle this without parsing the statement on the client side.