There are two fields on the Interactions in Team Approach that tie it back to a Prospect Plan in CRM, BBEC_PROSPECTPLANID and BBEC_PROSPECTPLANSTATUSCODEID. Since these are custom sync-related fields, the standard Team Approach Merge process is not set up to process and move the data from these fields from the dupe account Interaction to the new Interaction on the retained account. Because these fields are now null, the Interactions sync back to CRM to add the Interactions to the retained constituent. Since the records are not tied to the Prospect Plan anymore, they just show up as Interactions.
There is no fix to reassociate these Interactions with their Prospect Plans, as the information has been lost. Users need to manually create new Prospect Plan Step to replace the lost ones.
For a fix to prevent the issue from continuing in the future, contact TA Support.