Action buttons not displayed in email approval notifications

Summary:

Action buttons aren't being displayed in email approval notifications - all the user sees is

Content (required):

We're using the standard invoice approval template, and the email that's sent doesn't include the action buttons, though they are included in the template.

Checked already:

1. The notification is actionable
2. BI Publisher settings allow html to be rendered
3. Outlook allows HTML to be displayed

Version (include the version you are using, if applicable):

Fusion Applications 23B

Outlook 365

Code Snippet (add any code snippets that support your topic, if applicable):

Any advice appreciated

Thanks

Niall