The way postings are mapped into COPA value fields occurs in several configuration transactions. To get you started there is an analysis transaction that displays the mapping for you. Run transaction KECM, click on Value Field Analysis on the left side of the screen, select all the check boxes in Flows of Actual Values on the right side of the screen, press the Execute icon and enter through an information screen No account determination tables have been maintained if one appears.
You'll be presented with a screen showing six lines corresponding to the six check boxes you selected on the initial screen. These represent the six different types of ways that postings can be mapped to COPA. Now start exanding the hierarchy for each of the six lines individually and try and figure out which is the value field with the incorrect mapping. Hopefully, if your mapping is set up through cost element groups, you can just update the incorrect cost element group with transaction KAH2.