The invoice defaults to General Sales for new lines.
Did you use an inventory item? These can change the category itself.
If you are able to supply an invoice number, I can have a look at it for you and see if anything stands out?
Edit
I've had a look at the invoice you last edited which is food related, so please correct me if that's not the right invoice.
There was an edit on 31 January 2016, 11:16 which changed the nominal codes that the food and drinks lines were assigned to. They were changed from 4001 and 4002 to 4000 (General Sales):
