Anyone can check whether an e-invoice is on the tax system. That check is on the official General Department of Taxation portal, not inside Invoi.
Open Tra cứu hóa đơn điện tử.
You usually need the seller’s MST, the invoice series (ký hiệu) and number, and the issue date. If you have the CQT code, you can look it up that way too.
A match on that site means the invoice was accepted. No result — or details that do not match — means you should not treat it as issued.