The Flex ID can be displayed in a Display form and List View but not in a Sub Grid. To get around this limitation create a calculated column based on the ID and display that in the Sub Grid.
In the example below we created a calculated column in the Opportunity list for the CompanyID and called it “Acct ID”.
The “Acct ID” field will be available to the Sub Grid and has been added to the grid columns as shown below.