This isn't something wrong with your blend mode. It's an optical illusion called Mach Bands.
When there's a sudden change in brightness, the change in perceived brightness is exaggerated in the immediate vicinity, making it look like there's a dark line or shadow in the image even if it's not really there.
We can fix this by smoothing out our gradients so they don't have these sharp discontinuities.
Both the top and bottom orbs in the image below use the same additive blending calculation:
The orbs at the top show the same dark ridges as in your example, but the ones at the bottom don't. The reason is I reshaped the gradient slightly in Photoshop, to give it a little "ease out" at the dark end. Like this...
That's enough to smooth out the sharp change in brightness where the gradient bottoms-out to black, so we don't perceive a sharp ridge/dark trough there.