Are you using material Ledger?. If yes then the system must have been set the Material Price Determination Control to 3; this will ensure that Material Price is S; When this setting is applied, the Price & Exchange Rate Variances are loaded back to material at period End.
At period end, you have to execute the tcode CKMLCP; Suppose current month is Jan. on 31st Jan, you will close Jan and open Feb [both Logistic and Accounting]; then you have to execute CKMLCP for Jan; The Price/Exchange Rate Differences as accounted in Jan are bifurcated in to Consumption and Closing Stock and System will calculate PUP for Jan and revaluate the Consumption and Closing Stock for Jan.
If you are not using material ledger, then consider reversing the price difference documents via tcode MR22 for the previous period. Price differences do carry forward in subsequent periods.
