CCSID - RPG issues

Hello sages

I would greatly appreciate advice regarding the following CCSID issus:

We have lately added a 128 long, CCID(1208) description field to an
existing CCID(424) "mapping file" that maps external SKUs to our SKUs.
This file comprises mainly of:
EXTID - External entity ID
EXTSKU - External SKU
EXTDESC - External Description
INTSKU - Internal (Our) SKU
and the new
ITEMNAME8 - External description 1208

now,
I need to update the new EXTDESC1208 field using descriptions retrieved from
another file that has a CCID(1208) description field.

I have managed to do it with SQL but I would very much like to
do it in the original RPG program that populates this map-file.

after consulting the enlightening doc by Barabara morris

https://www.ibm.com/support/pages/system/files/inline-files/working_with_unicode_in_rpg.pdf
I came up with a skeleton semi-hard-coded RPG program that should be capable
of updating one specific record, BUT FAILS.

Here comes (forgive the mixed free/fixed format):

H DATEDIT(*YMD) ALWNULL(*USRCTL) CVTOPT(*VARCHAR : *VARGRAPHIC)

F* the source file to retrieve the description from
FNMMLIMPF2 if e k DISK EXTDESC('URPS/NMMLIMPF')
F EXTFILE(*EXTDESC)
F DATA(*NOCVT)
F
RENAME(NMMLIMPR:NMMLIMPR2)

F* The map file to be updated - target
FNMWRKF uf e k DISK DATA(*NOCVT)

dcl-ds ds_NMMLIMP2 likerec(NMMLIMPR2) ccsid(*exact)
dcl-ds ds_NMWRKR likerec(NMWRKR) ccsid(*exact) ;

// Get record to be updated
EXTID = '053' ;
EXTSKU = 'A3507531197482438BTRRtbKgG8' ;
chain (EXTID : EXITSKU) NMWRKF ds_NMWRKR ;

// Get CCID(1208) Description value from source file
SRCFILEID = 36694 ;
CATLOG = '8BTRRtbKgG8' ;
chain (SRCFILEID : CATLOG) NMMLIMPF2 ds_NMMLIMP2 ;

// take first 128 chars of Item Description
ds_NMWRKR.ITEMNAME8 = %subst(ds_NMMLIMP2.ITEMNAME :1 : 128) ;

// Update map file - target
update NMWRKR ds_NMWRKR ;

C eval *INLR = *ON

The update works but the value placed in ITEMNAME8 is erroneous

Now,
(green screen) Debugging this skeleton I suspect that the %subst(xxxx)
fails
but I may be wrong (not my 1st time)

The (source) NMMLIMP2.ITEMNAME field is a 512 long, nullable, CCID(1208)
VARLEN(30)
The (target) NMWRKR.ITEMNAME8 field is a 128 long not nullable
CCID(1208)

Any help will be greatly appreciated.

TIA
Gad