So a user places a bounty on a question but has no interaction with that question. It looks like they can be the focus of a comment but they do not appear in the reply autocomplete.
Considering that this person now has just as much of a stake as the original poster you would think that they would appear in this list but they don't. You have to type their full name. This isn't intuitive and would lead a normal person to think you can't comment at the bounty giver.