Sprite packer + 9-slice + alpha = bug

Using Unity 2018.2.5f1

Our UI artist created a button with a large glow effect that fades alpha out quite far.
This button is also integrated with the 9-slice feature so it can properly scale horizontally.
I created a sprite atlas and added our UI elements including that particular button.

When I look at the atlas preview, it seems fine. At runtime, that button bleeds a lot and displays some garbage at the bottom. I tried to increase the padding to 8 but it has little to no effect.

After investigation, I came to the conclusion that this might be a bug in the sprite packer. Maybe there is a hidden process that evaluate where to crop the pixels based on an undefined alpha threshold.

→ The issue is that the new height of that image doesn’t match with the height specified in the 9-slice settings of that sprite.

Get sure there is empty alpha on each edge of the image.
If there is one side touching the end of the image size it creates this issues.

That is normal behaviour if your Texture isn’t pixelperfect and there are no margins