Hi,
Please check the link https://scn.sap.com/thread/740123 regarding warning message when you enter the WT in basic pay.
For BF arrears:
How did you enter the wage type in IT8 (what is the date validity?).
Is there any INVAL entry presents in v_t539j for the new wage type which creates BF arrears from back date?
Please show RT table & output of WPBP in payroll log (because, payslip cannot explain the detail). Also please show the log of retro active accounting (if it triggers).
regards,
Bala