Answer by fluxtendu for Can't get my mail.app filter I created to work on messages that are coming in to the Gmail IMAP spam folder

I think the spam detection of gmail is a filter applied before others...

How to deal with it:

• To filter mails flagged as spam: add label:spam in the "Has the words" input box.
• To avoid the spam flagging, create a filter that describes the cases when Gmail should not flag the incoming messages as spam and then check "Never send it to spam".

source: googlesystem.blogspot.com