Plz recheck by running payroll in display log mode. Comparing when run Feb in Feb and Feb in Apr, as u said Feb in Feb is correct and retro Feb in Apr is wrong,
Plz check input of XNAB and output carefully. You can modify your schema using Function Print to display any thing you want (RT, IT, CRT, AB etc) before and after XNAB to verify. if same input in Feb in Feb and Feb in Apr, but different output (Feb in Feb is correct 100%), so I think some thing wrong with Function XNAB.
We have to contact to SAP by sending OSS message.
Regards,
Woody