Solution : https://service.sap.com/sap/support/notes/67641 (SAP Service marketplace login required)
Key words : 
additional key words ko88, attached source code change, past fiscal years, shortened fiscal year, settlement determines amounts, previous year, amounts settled, full settlement, completely settled, special periods
Related Notes : 
       
| 672255 | Shortened fiscal year/changing fiscal periods | 
| 130869 | Settlement rule: Validity period for shortened fiscal year | 
| 121478 | Settlement/overheads: in special periods | 
| 38285 | KD206 when settl.in spec.period of short.fisc.year |