Adding glow effect darkens image?

I added a glow to white text and the glow made the region slightly darker. If I increase the intensity, then I get a bright white centre but the area surrounding the centre is darkened which is also undesirable. Why would the glow effect produce a darkening?

The left 5 in the image below is the original and the right 5 is with the default glow effect added. The effect is subtle but it's darkening and not the expected lightening.


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