I'm pretty sure that previously I could add for example @Joe to a task by typing in "Joe" in the "CC" field, and it would be the default match, i. e. typing "Joe" RET would do what I mean. Now a list of five users is displayed whose real names match "Joe", with @Joe being none of them. Typing in "@Joe" does not list any users at all. On the other hand, typing in "scfc" instantly brings up my user account, despite "scfc" not being part of my real name.
It's probably futile to try to come to a consensus what a simple "Joe" should match against by default; however I think "@Joe" should unambiguously match against user names starting with "Joe", and "@Joe" RET should select @Joe.