By opening the Package.appxmanifest file, you can see the 'Package family name', which has the value of 'maui-package-name-placeholder_' followed by that string of seemingly random characters.

The Package family name is not editable - but when setting the Publisher details to point to the certificate I'd been using for my Xamarin app, the seemingly random characters changed to be the same as my Xamarin app.

Thanks to @BhavaneshN for pointing me in the right direction!