F107 - Flat-rate individual value adjustments

Dear Experts,

I am setting up the program F107 for my client to automatic valuate the debtors open items, the rule for valuating the open items is check if the item exist 180 days, if this condition fulfill then 100% of the invoice amount excluding tax will be valuated.

However, the result come out to some of the items are being valuated correctly but some items appeared in the way that partial tax were included in the valuation amount. I could not figure out where it goes wrong.

Anyone ever encounter the same problem? Can anyone advice where should I look into to identify the inconsistency of the system behavior?

Thank you