A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Hi respected AAA_013,
Thanks for your valuable feedback and sharing updates with us and I really appreciate your effort and your precious time doing.
And I appreciate your kind words.
If new added document library webpart still it doesn't work, then it seems that the problem happened with your Office 365 environment/Tenant level. It may need to scan/investigate from the backend side via remote section and collect 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, we sincerely recommend that you Microsoft 365 admin, can follow the steps in this article and open/create a support ticket to contact online support. The support team there has more permission than us, they can help you remotely and collect logs. They may also involve more resources to investigate the issue from the backend. This would be the most efficient way to handle this case for you.
I'm really Sorry and I apologize for it.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Waqas Muhammad