A lot of messages in MAL with "suspected spam" as part of the subject have been found without any spam action.
Check the problematic message from the Message Audit Log perspective if Spam or Suspected Spam can be found as verdict or untested verdicts. Gather the policies involved.
Double check the actions assigned to the involved policies. Based on the issue, the keyword "[suspected spam]" must be added by any other device than SMG as in our SMG we have the policy for suspected spam adding "[Suspected Spam]" and then to move the message to the quarantine.