This issue we have due to Listview rules and found its Pega product issue ,Please refer the below details
ISSUE : Data Instances were not included in product jar when a Developer included ListView Rule with certain Filter criteria in Product Rule to extract Data Instances. When a listview filter is added to RAP, the instances returned by listview must be packaged by Rule-Admin-Product, but this was not happening if the listview filter did not have pyRuleSet in its fields. When a pyRuleSet is not included in listview, records returned by listview filter will be treated as invalid by export process in InstanceInfo class. To avoid unexpected behavior, .pyRuleSet will now be added to fields of listview filter if it doesn't exist.
You ask: "is it possible to export external table data in a RAP"
Please find the existing documentation abut using Data Types rather than Data Tables. Data tables creation was deprecated from Pega 7.1.7+ onwards and we encourage to use Data Types as a replacement of it, which can be created from Data explorer tab.