sympl-mail: A default-forward configured for a domain bypasses SpamAssassin filtering
As mentioned: https://forum.sympl.host/t/spam-not-being-tagged-nor-moved-to-the-spam-folder/63/4?u=kelduum
Seems likely its an order of execution thing - the mail is being handled by the delivery function before it's been scanned.
IIRC, this was an issue with Symbiosis as well, so likely has been around a while.