Solution : https://service.sap.com/sap/support/notes/159412 (SAP Service marketplace login required)
Key words : 
shift-jis front-end, map normal unicode katakana, half-width katakana typed, x'df'full-width, explicite half-width characters, shift-jis reason, half-width katakana, round trip compatibility, higher memory consumption, double-byte characters
Related Notes : 
       
| 938738 | Syslog CP6: still using ambiguous blended code page | 
| 938737 | Syslog CP6: still using blended code page | 
| 331646 | Missing Japanese characters in UTF-8 |