I figured this out late yesterday, after my previous post. I'd noticed that a new "Barcode Reader" control had been released for preview on the 19th October and I'd wondered if there was a connection - and there was!
It appears that the current "Barcode Scanner" control no longer has the ability to output non-printing ASCII characters, like the character #29 'Group Separator' that is encoded into GS1 barcodes. The new "Barcode Reader" control has this ability, so with a little reworking of the splitting code:
...and replacement of the old "Scanner" controls with the new "Reader" controls, full functionality is restored.
I'm a little bit baffled as to why the ability for the current "Scanner" control to output non-printing characters has been depreciated whilst the new "Reader" control is still in preview mode. Is there an underlying code module that can only be pointed to one control at a time?