Messages only displaying one message per person

[Avatar-1]

Clicking on any of the message threads in my inbox or sent messages, it's only displaying the first message I've sent or received from that person.

By "thread" I mean with that same subject line.

[Gary Teter]

We are unable to duplicate this problem—it's occurring with everybody or only with messages to/from some people?

[Avatar-1]

Everybody, as far as I can tell.

The second last digit in the URL changes, though the message doesn't - I can open two messages, and see a 2 in one URL and a 3 in the other URL.