Giants and supergiants actually tend to have smaller habitable zones. While their temperature and luminosity are higher, there are also more lethal levels of radiation.
I'm not sure if these zones accurately depict this phenomenon.
If you're using a main-sequence star, this might be a bug. The system might have trouble differentiating between a large main-sequence star and a small giant star.
Oh, I see. I guess smaller habitable zones on larger stars do make sense, then. My knowledge on this area is still pretty limited.
The example in the attached image is just a random star I slotted in from the "Random Known Star" tab. As for whether it is a main sequence, I'm not too sure. However, as you said, if I do place down a main sequence star, say, from the "Random Main Sequence Star" tab, and simply up its mass beyond about 3 solar masses, the display issue occurs. You can try to replicate the problem that way.
If this is in fact an accurate representation of habitable zones on larger stars, I find the transition a little strange. Once the mass of the star reaches the critical point of 3 solar masses or so, the "normal" habitable zone of the red, green and blue hues instantly disappears, and is replaced by new display shown in the image I attached at a
much further distance away from the star. I have to zoom out a fair bit to see it. Is this sudden jump correct?