Alpha Glow cut off around portions of transparent PNG

[longhaulpro]

See the screenshot and note how the glow around the PIX is weirdly boxed in - can anyone explain what's happening, and how to fix it?

I thought this had to do with the canvas size of the image, so I made it considerably bigger. No change. This has happened to me repeatedly so I'd be grateful to know how to avoid it in the future.

[longhaulpro]

(To be clear, I'm referring to and using the Alpha Glow effect).

[Richard M Knight]

Try turning off 'Composite in Linear Colour' in the sequence settings