Mail rules keep changing

I use rules in Mail to sort incoming mail into various Mailboxes "On My Mac". I am also training Spam Assassin by copying non-spam emails into a scan-ham folder that is on the IMAP mail server.

So an example rule would be:

If any of the following conditions are met:

From contains emailaddress1

From contains emailaddress2

From contains emailaddress3

Perform the following actions:

Move Message to mailbox: Mailbox1

Copy Message to mailbox: scan-ham

However, what I have been seeing, is that sometime later, the last section of the rule has changed to ...

Perform the following actions:

Move Message to mailbox: Mailbox1

Copy Message to mailbox: Mailbox1

The scan-ham part of the action has been replaced. Why should this happen?