I’m assuming that you have two tax boxes appear on the new/edit invoices screen and that you have a tax such as GST selected when you save it.
Also that you are running the most current version of SI master 2020 loaded. You should see “Version: 2020.3.13 — 20211104” in the About box. I ask specifically about this as I fixed this issue (not sure how long ago) where this was happening. So make sure your version is current.