My guess would be that whopper of a Fidelity setting.
From AI help:
Fidelity can range from 0.5 to 20 pixels; the higher the value, the smoother and less complex the path. Controls the amount of smoothing that Illustrator applies when you use the tool.
I absolutely understand the confusion and why a high Fidelity setting may seem ...
This is how Illustrator works. You are creating "Objects" with your brush and they are listed in the Layers Panel separately. That way they can each have different appearance attributes (different fill and/or stroke colors, width profiles, etc.). You can simply twirl the parent layer closed if this is bothering you or create a "Sub Layer" ...
There is. For some inexplicable reason it is hidden inside the 'Dynamics' options, whose help page is here. One of the preset dynamics you can select is 'Color From Gradient', and it does exactly what the 2.2 dialog describes with a slightly different UI.
... use the Pencil Tool (Under the Brush Tool) for 1 pixel.
Brushes assume some smoothing is desired. The Pencil Tool won't.
Unless there's some reason I can't fathom, I don't know why anyone would attempt to define a 1px brush.
It looks like this has something to do with using the "Soft" mask option in Krita. I tried to replicate this in other applications such as GIMP and Photoshop, but failed. So, it seems to be a peculiarity of Krita's brush engine. As for the reason, perhaps reach out to the Krita developers. I suppose it will depend on the maths that lies behind the ...
ADD1: This elementary transparency thing isn't the cause, see ADD2.
Dropping linearly = increase transparency linearly. You expect that at some point, say at 50% point of the slope the brightness should be full 100 % if there's in the same place also a 50% point of the lower light layer.
But it isn't. If the lower layer is also 50% transparent there can be ...