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Somehow I think this because of the filter dimensions, the extended parts are cut. Try extending these boundaries:

<filter id="degenCodeNeon" x="-50%" y="-50%" width="200%" height="200%">

The percentage uses objectBoundingBox, you could have also specified userSpaceOnUse. But first try this one.

Tried with x,y -250% and width/height 600%, seems to work. I suggest adding color matrix or component transfer filters as addition to completely reduce alpha to 0 below a certain threshold.

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