Sort by "Who" column not chronological

We recently upgraded to Domino 9.0.1 and one of our users is now complaining that when sorting by Who/Sender column, the emails in her Inbox are not sorted chronologically. I verified this by checking her mail DB on my own client and the same problem occurred, so I don’t think it’s a Notes client issue. It doesn’t seem to be happening to anyone else that I know of, as sorting by Who typically shows things chronologically for each sender. Is there a setting I’m missing? Thanks.

Subject: Try replacing mail file design…

If that does not work try opening the mail file in designer and see if someone has done any customization to the Inbox and added prohibit design refresh…

jpaganet@us.ibm.com