Hello @Kiều Quốc Hưng,
Thank you for reaching out to the Q&A forum!
I hope you understand that this forum has limitations in terms of available tools and testing environments. As a forum moderator, I only can try to do research on the issue and provide guidance based on my experience.
However, I’ll try my best to help point you in the right direction.
Since the issue started right after the December update, I recommend you reach out this issue to our higher-level technical support team for backend investigation.
You can ask the customer to contact their organization’s IT administrator and raise a support ticket through the Microsoft 365 Admin Center.
- To create a Support ticket, the IT admin can go to: Microsoft 365 admin center
- When they click Help & Support in the bottom-left corner, a Support Assistance panel will appear.
- In the text box, they can type: "request an agent to contact me via email or phone" and press Enter.
- After that, please look at the bottom-left corner of the panel. They will see a blue link that says, "Contact Support"; you click on it.
- They will then have two options: Support via phone or Support via email. You can choose Email.

Please follow this article: Get support - Microsoft 365 admin | Microsoft Learn
When the ticket is opened, a support engineer can start a remote session to investigate the issue, review the back-end settings, and optionally run a sync tool to resolve the issue.
I truly hope you’ll be assisted soon by our dedicated technical team. And if you receive any updates or insights, please feel free to reply here.
Looking forward to hearing from you!
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