All Excel Macro Files Suddenly Flagged as Malware (X97M/Slacker.gen!A) Across M365 Starting April 16

Starting around 8 PM GMT+8 on April 16, 2025, macro-enabled Excel files with extensions such as .xlsm, .xlsb, or .xls began being automatically flagged as malware, specifically identified as X97M/Slacker.gen!A—when opened or edited in SharePoint, OneDrive, or Teams. Before this, the same files were not flagged as malicious, even when opened or edited, and this behavior had remained consistent for several months. 

This issue affects our entire tenant, with over 800 files being flagged as malware under the name X97M/Slacker.gen!A. These files are located across various locations and have been modified by different users. 

We are a Cloud-only tenant, and we have not done any configuration changes in Threat Policies for the past few months.