The term ‘flare’is a garbage term. It’s totally bokeh.
The reason it’s triangular is because it’s a three leaf iris diaphragm imaging an out of focus point source. Those have only become common more recently.
The shape of the bokeh is driven by the shape of the partially open diaphragm or the shape of the aperture mask. If the aperture is full open the bokeh would be round the shape of the lenses. Stop down a more traditional 6 leaf iris diaphragm and you’ll see something more hexagonal. Not fully hexagonal because the inner edge of each leaf is slightly concave to more approximate a circle and to minimize Diffraction effects. In fact look at the sides of the triangles on the UFOs and they are slightly convex. Again that’s built into the diaphragm leaves to minimize diffraction. Put a square aperture disc if front and point your fully open lens at an out of focus star and you will get a square bokeh.
Feel free to doubt me but I’ve worked with optics all my life. The triangles are caused by the shape of the iris diaphragm pointed at an out of focus point source. I’ll bet my life on it.
The explanation is very plausible because that’s what it looks like, at any rate.
