scratching my head on IMAP and MIME messages

I’ve found a rather odd behavior: sometime when I receive MIME-encoded messages (INCOMING message), only the FIRST part of the multipart message is sent to an AppleScript script attached to a rule.

BUT, after the message is downloaded, if I do “Apply Rules” on the same message, ALL parts are sent to the script.

This is observed with an IMAP account. Has anyone observed this? Maybe some bandwidth-preserving thing is going on here? Is it documented?

If it’s the case, this is really annoying cuz for some task (like a complete classification whether a message is junk or not), we need COMPLETE messages.

Hope someone could share some insight here, thx a lot.