I have exactly same thing. I have never enabled the “Untrusted” or added any passwords. I have no idea where did that come from.
EDIT. When I added another folder from another computer, “Receive encrypted” was the default option.
EDIT2. I found the problem and it is not directly Syncthing required. I had “auto-fill” on in my password manager and it recognized the Device->Sharing fields as password fields and added the passwords when I did other edits. Maybe Syncthing could enable the fields only if a device has been set “Untrusted”?