It looks rather strange, as if after fading the outer glow starts gathering strength again, visible at the corners of the Bounding Box of the glow effect.
What happens if you close down Illy and Move the folder (you may move it back to reset your preferences)?
Maybe mayb not there seems to be a bug of somekind look at these ssettings particularly att the one for 30 blur athen look at the others, notice that at 30 it appears and at 32 or 29 it is gone and it does not matter how much extra added space I give it 30 will produce the artifact.
Same settings at 40
So try changing the setting of the blur to a different number like add or subtract a pt.