The Sun beams node produces sharp results when the background has an
alpha channel. That's because the sun beams samples were weights by the
alpha channel, and when the image is mostly transparent, no smooth
falloff can exist.
This is a regression in the Sun Beams redesign that took place in v3.6.
I was mislead by the fact that the alpha was multiplied to the weight.
But the original code actually multiplied the alpha of the border color,
not the sample color. So this fix essentially restores the old behavior.
Pull Request: https://projects.blender.org/blender/blender/pulls/124532
acd4a85d6b