There are no tagging suggestions when writing an private conversation.
At first I thought it could be by design, but when I sent the message I saw that the username I tagged got linked to it profile. So if the tagging is activated in conversations, the suggestions should be working I guess.
I have a feeling this might get marked as designed, since the user can't be notified.. but the usefulness of tagging extends beyond the notifications so I'd like to see suggestions in conversations as well.
It was changed explicitly. In contexts where an alert isn't triggered, we don't offer auto-complete as it removes one of the hints about tagging. We do still keep the automated linking as that can be useful.