User is trying to create an invoice for a client, but the actual contact and address details of the client are not showing up on the invoice and are getting replaced by a different contact and address.
This can happen if you have set up a Billing Contact at the project level and checked the option Send Invoice to Contact in Projects > General > Billing Options. In this case, Core uses the details of the Billing Contact on the invoices. If you want Core to show the original contact details of the client on the invoices, un-check the Send Invoice to Contact option for the project. You might also want to check out these articles: