As far i understand with reference to your screen shot.
Please find the below calculation
/AP2 of May, June, July ,August (-7412.44)+(-12094)+833+833 = -17840.44 /AP2 this will be getting added to /ZP2 i.e /ZP2 = -17840.44 in current month September /ZP2 will get added to /3P2 which is 12927 so 12927-17840.44 = -4913.44 is your P Tax basis for current month so you have to find out why the value /AP2 is getting populated in table for that go to log and check for the month of May,June and check the PCR IN42 Input, Processing, Output so that you can resolve the issue.
Your basis value is negative so system is not calculating P Tax as suggested check the log of May, June thoroughly there only you can trace out why /AP2 is getting generated.
Regards
Venkatesh