In order to have both a Lead and a Contact field sync with the same Marketing Cloud Account Engagement prospect field, two requirements must be met. They are:
The Lead and Contact fields must have the same API name. The API name is the unique identifier of a field in Salesforce, and it must match exactly between the Lead and Contact objects. For example, if the Lead field has an API name of Lead_Source__c, the Contact field must also have the same API name4 The Marketing Cloud Account Engagement field must be mapped to the Salesforce field. The mapping is the connection between the fields in Account Engagement and Salesforce, and it allows the data to sync between the two systems. The mapping can be done manually or automatically, depending on the field type and the connector version