I use Apple Mail. I recently carried out a Software Update to Mojave (macOS 10.14.6 Supplemental Update released Oct 1st) and immediately noticed that emails that had long been routed to the Spam folder by SpamSieve were appearing in my inbox. Yesterday, the 5th of Oct., I checked in Mail>Preferences>Rules only to find that SpamSieve (top of the list) was unchecked (as were others).
Adding the check back sorted the problem.
Thought I’d pass this on.