High Pass filtering 2.8 vs. 2.10

[Espermaschine]

The manual way of making a High Pass Filter (that is also used in RobA's script) is:

1. duplicate image
   
2. desaturate
   
3. invert
   
4. reduce opacity to 50%
   
5. gaussian blur
   
6. merge down
   

When you do the steps in 2.8, the image becomes 50% gray at step 4.

This is not true for 2.10, unless you set the Blendmodes to Legacy.

I know Gimp 2.10 has a GEGL High Pass Filter, but i want to understand the "wiring under the board".

The only 2.10 procedure i can up with is:

1. duplicate image
   
2. desaturate
   
3. invert
   
4. set layermode to 'Grain Merge'
   
5. gaussian blur
   
6. merge down
   

So what is going on ? What has changed that makes the old way dysfunctional ?