It kind of defeats the meaning of "selflessness" to say any reward for such an action makes it selfish. I think the word was created to distinguish between those who go out their way to help others from those who don't.
I understand the point about positive feelings, however I think it undermines the language we are using. Consider that any conscious rational action could be reduced to "for the positive feeling". So the person who loves to paint, actually loves the positive feelings, as does the person who loves to hunt. In the end, separating what causes the feelings from the feelings themselves isn't a very useful distinction.
However, I can think of a whole bunch of exceptions or weird cases for a thought experiment regardless:-
1. Someone can either dedicate their life to charity work, or to rising up the ladder of investment banking. In either case they will receive equal levels of happiness, yet the person chooses the former (emotionless reasoning or some such). Selfless choice?
2. Instinctive actions, reflexive actions, and those without motivation. Can they be selfless?
3. People who do things to help others, who are either obsessed with it or addicted to it. Seems like no actual desire is necessarily at play.
4. "Wrong decisions". You may have played a game of some sort, and simply made what you see as a "wrong" decision, whether you knew the "right" one or not. Most often in the heat of the moment, but it happens in turn-based stuff as well. So, part of you went against whichever other part thought it was "wrong". Just copy and paste that onto taking a bullet for someone. It may be related to instinctive actions, I don't know.
5. People forced into actions that serve others. So the person who loves to hunt, becomes one who has to hunt. You can say they "want" to do it, fair enough, but on some level they don't want to, as they have to be forced into it. It could be something as simple as feeding your family. Is it truly always the case that they get positive feelings from doing it?
There's also the idea that saying people do things because they want to do them, says more about the English language than reality. Since there aren't many methods of expressing why someone does something without throwing the vague terms "want", "desire" and similar out. In actual fact, those terms may rarely refer to qualitative feelings, and just be an aspect of language.