A search for a good explanation of why linear mode affects transparency like that - nothing so far. There are users who complain that the Gimp 2.10 GEGL gaussian blur is too different from the Gimp 2.8 blur.
Using Gimp 2.10 there is another way to apply that effect, the new drop shadow filter.
Using Gimp 2.10 there is another way to apply that effect, the new drop shadow filter.