This is not really a bug, but it is undesirable and counter-intuitive behavior.
We just had a customer that was pretty much panicking because it looked like users were seeing topics in the email digest that they shouldn’t see. It turned out that if the username autocomplete is not selected, the username is filled in into the username field, but is not being used.
This is not a single incident, in the past months we had at least five reports of strange behavior that could be pinned down to the administrator not confirming a username autocomplete. This time it was the digest preview, another incident involved adding users to a group.
Here is a screencap that shows what I mean. It involves user Richard who is a regular user, but who seems to get a topic in an admin-only category in his preview digest.