Gwenhywfar won't correctly read a UTF-8-encoded MT940 file.
#1 Updated by firstname.lastname@example.org 7 months ago
Please see https://bugs.gnucash.org/show_bug.cgi?id=797050 for details and a sample file.
The GnuCash calling code is at https://github.com/Gnucash/gnucash/blob/maint/gnucash/import-export/aqb/gnc-file-aqb-import.c#L71. AFAICT the file is passed directly to Gwenhywfar with no transcoding.