Syntax Issues while Importing data into Salesforce from S3 Bucket Using Amazon App Flow

Created an App flow to Load Data from S3 Bucket to salesforce Instance. While mapping Standard Look Up fields AppFlow is using wrong syntax for field Mapping due to this I am getting Error while loading data .

Example : During Contact Upsert while relating contact record with Account using Account External ID, Appflow is using Syntax as “AccountID.AccountExternalField”, but it should be “Account.AccountExteranlField”.

Error Message :

[{"error":"[{"message":"Cannot deserialize instance of reference from START_OBJECT value { or request may be missing a required field at [line:1, column:149]","errorCode":"JSON_PARSER_ERROR"}]