Hello Leong,
Good day. Thanks for posting in the Microsoft Community.
Here is a recent service incident you may check first in your admin center > health > service health >SP1063758 - Users search results in SharePoint Online may show empty search results
The root cause for that: A high number of app requests and retries to an API that facilitates SharePoint Online search is resulting in the impact.
Not sure if this will also affect "Update Item Action" in your flow. You also submit a ticket to online technical support team from https://admin.microsoft.com/#/support/requests, they will have higher permission than us to investigate the issue for you.
Hope this helps.
Best regards,
Doris V - MSFT | Microsoft Community Support Specialist