Oracle Datapump API - APPEND - oracle

Question

The following tries to APPEND data to the CLASS table from a dmp that contains several hundred records for that table. There is 1 record already in database and that record also exists in the .dmp. The job completes but no data is appended.
If i switch the parameter to TRUNCATE, the single value is removed and replacedw ith the content of the .dmp i.e .works correctly.
Can anyone tell me what's wrong with this proc that's causing APPEND to fail?
The documentation doesn't seem to offer much beyond a reference to metadata (https://docs.oracle.com/database/121/ARPLS/d_datpmp.htm#ARPLS66059)... i have tried removing the metadata filter included in the code below, but with no +ve result.
Thanks
create or replace procedure import_backup (in_file_name in varchar2,
in_job_name in varchar2,
in_export_filename in varchar2)
is
l_dp_handle number;
l_status varchar2(200); -- Data Pump Status
l_file_name varchar2(100) := nvl(in_file_name,'IMPORTBACKUP');
l_job_name varchar2(100) := nvl(in_job_name,'IMPORTBACKUP20');
l_ipaddress VARCHAR2(20);
l_time VARCHAR2(20);
PROCEDURE_NAME VARCHAR2(30) := 'export_backup';
begin
select PARAM_VALUE
into l_ipaddress
from PARAMETER
where PARAM_NAME = 'DATABASE_SERVER';
select to_char(sysdate, 'YYYYMMDDHHMISS')
into l_time
from dual;
--Log_Message(PROCEDURE_NAME,'Starting with in_file_name: ' || l_file_name || ' and in_job_name: ' || l_job_name);
-- Open a table export job.
l_dp_handle := dbms_datapump.open(operation => 'IMPORT',
job_mode => 'TABLE',
remote_link => NULL,
job_name => l_job_name,
version => 'LATEST');
-- Specify the dump file name and directory object name.
dbms_datapump.add_file(handle => l_dp_handle,
filename => in_export_filename,
directory => 'EXPORT_DIR');
--Specify Truncate mode (to remove table contents and replace with file's contents
DBMS_DATAPUMP.SET_PARAMETER (
handle => l_dp_handle,
name => 'TABLE_EXISTS_ACTION',
value => 'APPEND' );
-- Specify the log file name and directory object name.
dbms_datapump.add_file(
handle => l_dp_handle,
filename => in_export_filename || l_time ||'expdp_XYZ.log',
directory => 'EXPORT_DIR',
filetype => DBMS_DATAPUMP.KU$_FILE_TYPE_LOG_FILE);
-- Specify the table to be imported, filtering the schema and table.
dbms_datapump.metadata_filter(handle => l_dp_handle,
name => 'NAME_EXPR',
value => 'IN (''CLASS'')',
object_type => 'TABLE');
-- Exclude table stats
dbms_datapump.metadata_filter(handle => l_dp_handle,
name => 'EXCLUDE_PATH_EXPR',
value => 'IN (''STATISTICS'')');
dbms_datapump.start_job(l_dp_handle);
-- Wait for job
dbms_datapump.wait_for_job(handle => l_dp_handle,job_state => l_status );
dbms_output.put_line( l_status );
--Log_Message(PROCEDURE_NAME,'Completed DataPump Job');
-- Job will continue running the background until it completes or errors
--dbms_datapump.detach(l_dp_handle);
exception
when others then
--Log_Message(PROCEDURE_NAME,'Datapump job failed');
begin
dbms_datapump.detach(l_dp_handle);
execute immediate 'Drop table ' ||l_job_name ;
end;
end import_backup;
Have attempted using 1 record in exiting table and using APPEND - no results
Have attemped using 1 record in exiting table and using using TRUNCATE - the 1 record is deleted and replaced with the content of the dmp's CLASS table