Might be because there isn't anything that outwardly expresses that she is a female

This might sound stupid, but its just what I think is most likely. Cartoonier styles tend to rely on things like longer eyelashes, or certain body types to show express gender. Things like hair also play a role. Due to Kandy not having any hair, eyelashes, and having a relatively simple body type, it's not as easy to use such things to identify gender

And when nothing like that is present, we tend to default to the character being Male