Solution : https://service.sap.com/sap/support/notes/1340365 (SAP Service marketplace login required)
Key words : 
fiscal year variant, system inadvertently displays, day ledger reason, relevant support package, function module fagl_get_account_balance, terms fagl_get_account_balance, selected period, completely solve, cumulative balance, solution implement
Related Notes : 
       
| 1354809 | FAGL_GET_ACCOUNT_BALANCE no values delivered for period 0 | 
| 672255 | Shortened fiscal year/changing fiscal periods |