Can't figure out why this sender's emails are coming over as "spam" on the subject line when I can see it didn't send that way...it seems to be doing this on our end (not theirs). They are identified as safe senders, safe domain, etc....and it's happening intermittently. Thanks!

Outlook doesn't alter subjects when a message is being detected as spam. This is done either by a 3rd party add-in that you installed or a spam filter installed on your mail server.