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Dear chrischan,
Thank you for posting back. I appreciate your precious time doing tests and sharing with me the detailed information.
Considering that this issue persists even in different browsers, I'm afraid this issue needs to be investigated from the backend side via remote assistance and collect the background logs.
As this Forum community has limited resources, the backend support team there has more permission than us, they have more resources to investigate the issue from the backend, they have Escalate channel to escalate your issue to check more and collect log reports for the root cause.
In this situation, I sincerely recommend that your Microsoft 365 admin, can follow the steps in this article and create a support ticket to contact online support. This would be the most efficient way to handle this case for you.
Note: To create a support ticket, you need to have Microsoft 365 admin privileges, and if you're not an admin for your Office 365 organization, contact the admin of creating a new ticket.
Reference: How do I find my Microsoft 365 admin? - Microsoft Support
Please accept my apologies for redirecting you to the related team support, as the moderators in this community have limited resources to verify the backend information and to get the quick and better assistance, we requested for this.
Your understanding and patience will be greatly appreciated. I hope you're staying safe and well!
Sincerely,
Tina | Microsoft Community Moderator