Thanks a lot for the answer. 

I have checked all the above mentioned and it seems to be correct, however, I still don't see the fields in the Invoice. 

I am trying to add a 2 GLNs to the customer invoice and in the invoice creation view, it is all visible. As soon as I create the invoice and view it, both added fields are gone. 

Hi and welcome to the Odoo community! 😊

When you add related fields in Odoo Studio, especially in the Invoice configuration section (I'm guessing you're modifying the customer invoice form), there are a few things to check:

1. Make sure the fields are added to the correct view: Sometimes, fields are added to the model but not actually placed in the form view that's used when creating/viewing invoices. Try opening the invoice form in Studio, and double-check that your fields are placed on the form layout where you want them to appear.
2. Check field visibility settings: In Studio, select the related field and click on the "Visible" option. Make sure there's no condition hiding the field (like invisible domain conditions).
3. Verify model relationships: Related fields depend on the relationship path being correct. If the source field (the one you’re linking to) is not properly accessible from the invoice model, the related field won’t show any value—or might not appear at all.
4. Activate developer mode: Sometimes it helps to turn on developer mode (from Settings > Activate Developer Mode) to see more detailed info about the field or to inspect the form view.

Let us know what models you’re linking through the related fields, and we can help dig a bit deeper!