Hi Wayne,
To help isolate the behavior, test and see if the alert tone works with a different contact, and test if a different tone works for the affected contact. Additionally, see if the tone works for a different application. This will help to see if the issue is isolated to the alert tone, application, or contact.
If you need help with assigning this tone to another app's notification setting, then you can follow the steps below to do so (not all apps allow this setting to be changed): Use notifications on your iPhone or iPad - Apple Support
"Change alert styles
- Go to Settings and tap Notifications.
- Select an app under Notification Style.
- Under Alerts, choose the alert style that you want. If you turn on Allow Notifications, choose when you want the notifications deliveredâimmediately or in the scheduled notification summary.
With iOS 15 and iPadOS 15 or later, you can turn on Critical Alerts for supported apps so notifications come through even if your iPhone is muted or you have a Focus set up."
If the issue seems isolated to the contact, then test after deleting and re-creating the contact to see if that clears up the behavior. If the issue persists, then reach out to Apple Support directly to help investigate the behavior. You can use the following link to reach out: Contact - Official Apple Support
Kind regards.