Large number of messages from regular customers going to Junk

[ProsolUK]

In own organization messages from regular customers who we have been in touch with for months without problems have started going into Junk during the last 7 days. Many of our clients (who are likely to also be users of Microsoft 365) are experiencing the same issues at their end with messages from multiple customers suddenly starting to go into Junk within the last 7 days.

We have messages filtered through Spam Experts initially and then forward to Microsoft 365, with Spam Experts setup as a partner organization Connector and the Enhanced Filtering for Connectors for this Connector set to Off.

Junk protection on the Outlook Clients is set to "No automatic filtering".

Despite this however, important messages are STILL going into the Junk Folder.

This certainly seems to imply changes have been made somewhere within Microsoft 365 during the last 7 days that is causing this issue for many tenants.

How can this behavior be prevented, it is seriously affecting our day to day business operations, and it would appear effecting OTHER customers in the same manner.

[#2]

@ProsolUK what type of threat policies are configured in your O365 defender?

[#3]

Anti-Spam policy is set to lowest, so have changed "Move to Junk Email folder" actions to "Prepend subject line with text" across the board, to see if message stop going into Junk Email and end up with prepended subject lines.

[#4]

@ProsolUK from your anti-spam  policy add your domain to the allowed domains and see if your users will still be affected with this abnormal behavior