"Individuals who identify as heterosexual but engage in same-sex sexual behavior" are simply not heterosexual, by definition.
You can have sex with someone without being sexually attracted to their sex in general (with them being the exception) or even without being sexually attracted to them at all. Maybe you just want company while jerking off.
And while there are people who will call others "gay" as soon as they do anything that could be interpreted as vaguely sexual with someone of the same sex, that's hardly a scientific definition. And no one will identify themselves as this kind of "gay".
People do not have to be consistent. Three follow up study could try to determine how they identify themselves later and what kind of sex life these people have. The study is probably underpowered to detect any mental issue if any exists whatsoever.
Whether you're sexually attracted to men or women is entirely subjective and not mutually exclusive. You can have a preference, which is most likely what you identify as, but you won't find the other sex entirely sexually unattractive as a result of that.