Why don’t emails that show up labeled as [SPAM] in the subject actually GO to my spam folder? It’s obvious that eM Client is recognizing them as spam (correctly!), but why won’t it automatically move them to the spam folder?
eM Client does not change the subject of incoming emails. That might be done by your server.
Unless you have blacklisted addresses or domains, the only way that eM Client can move email to the Junk E-mail folder is if the Spam filter is enabled and the message contains X-Spam-Status or X-Spam: Yes in the header.
If those are not working for you, you can create a Rule to move emails with [SPAM] in the subject to the Junk folder.