In order to sync upsells back to Thais, you must locate the Item Code in Thais and add it back to the Billing Identifier section of the Upsell
Go to Configuration > Items
You can find the item setup and the Item Code number next to the service is the billing identifier -
Copy the Item Code into the Upsell. Go to Advanced and paste the Item Code into the Billing Identifier section
Save your changes and then you're done!