Dear @Anita - Sydney AUS,
Thank you for reaching out to the Microsoft Q&A forum. I am happy to assist you.
There have been user reports of similar behavior when opening Visio files using the SharePoint viewer or Visio for the web. Since your diagram is complex, some functions may not work as expected in the web version.
Try downloading the file and opening it in the Visio desktop app to help isolate the issue. Additionally, test it in a different browser to determine whether the behavior is browser‑related.
Additionally, here are some links you can review for alternative approaches: Limitations to file editing in Visio for the web | Visio Online : Hyperlinks to specific page not working in Sharepoint
If this behavior continues, I recommend submitting feedback to Microsoft so the product team can acknowledge the issue and consider improving the experience in future updates.
You can submit feedback directly to Microsoft via the Visio app (Help > Give feedback), or through Feedback Hub by
Go to: Microsoft 365 Feedback Hub > Send Feedback > Describe your proposal in detail.
Additionally, since your diagram is complex and each device or environment can differ in configuration, you may want to open a support ticket with Microsoft. The support team has the proper tools and permissions to review your actual file, investigate the behavior directly, and provide more accurate guidance.
The Global Admin can create a support ticket by sign in to the Microsoft 365 Admin Center > Support > Help & Support.
For detailed instructions on how to get support, please refer to Get support - Microsoft 365 admin.
Note: If you’re an end-user, please coordinate with your Office 365 Global Administrator to handle these steps.
If you need any further assistance, feel free to reach out. I am here to help. Thank you very much for your understanding and your cooperation.