Emails marked as SPAM

Software - Avast Business Anti Virus Pro 19.7.2573

Today we had an issue where a lot of users using outlook had the Subject of their emails modified to include SPAM even though they were not spam at all. Many of them were regular internal emails. During this period every single email of these users were modified this way. This occurred even though “Mark in subject of spam messages” was not selected (checked both in the console and the client application). We had to turn off the anti-spam feature through the console altogether to resolve the issue.

Also we use google (Gmail for Business) as our email provider with strict anti-spam settings. They didn’t mark these as spam.

Any pointers as to why this might have occurred?