Using Classic Outlook on Windows for personal email, calendar, and contact management
If you sign in as tenant admin to this page, does it match mine below, https://security.microsoft.com/securitysettings/userSubmission? Does your third-party solution need anything configured on this page? When I set this page to below and restart Outlook the Report button is gone.
I am using the guidance from here, Report phishing and suspicious emails in Outlook for admins - Microsoft Defender for Office 365 | Microsoft Learn.