If you create a connection to an external database, then a table connection based on the external database the Custom_Edits table will not auto populate with the fields from the external table.
Custom_Edits table is not automatically updated with the fields from an external table when the connection is created.
One potential workaround via the UI is to go to Administration->Customization->Secondary Entities-> <Your external Table> -> Fields
Edit and save each field and they should now be appended to the Custom_Edits table. You should verify this by checking the contents of the Custom_edits table:
select * from Custom_Edits where ColP_Entity = '<Your external table name>'
Two rows will get created when you create the external connection at first however they will only have null values. Once you edit and save a field it will also be added to the table.
This is a known bug and has been escalated to development.