I notice that in May payroll:
Total Payment is 60K
Tax is ~12K
=> expected Net pay is 48K
But Net pay is 54.3K. So the Net pay in May16 also includes the backdated increment in Apr16. Therefore the standard retro is working fine as usual.
If you want to show the difference between original Apr16 in Apr16 and for Apr16 in May16, you might select another option in Remuneration program:
FYI you might look at the document to understand how the standard SAP retroactive accounting work.
Understanding Retroactive Accounting
Hope it helps.
Best Regards,
Woody (Trong Minh)