Repro 1:
Create a new image
Fill the image with half-transparent white
Run Gaussian Blur at default radius; the whole image should noticeably darken
The image is noticeably darker. Check the color channels; they're definitely not white anymore
Before|After
Repro 2:
Create a new image
Delete a section in the middle of the image
Run Gaussian Blur at default radius
Repeat with Ctrl+F until you see edges darkening; should be noticeable on the 5th time, and painful on