UI atlas icon artefact

I'm building a UI that uses sprites packed using the Unity sprite packer. Sprites are mostly white with alpha, with colour tints being applied dynamically at runtime.

Some sprites in the UI will show an artefact of a 1 pixel line along one edge. When the atlas is updated and therefore repacked, the artefact may go away but appear on a different sprite in the atlas. On inspection of the 'Sprite Packer' it does not look as though the artefact comes from a neighbouring sprite.

I've tried adjusting the packer policy and the padding power, which does not seem to help.

Has anyone come across this problem? Any suggestions?

Here is an example of the artefact:

And here is the icon in the sprite packer: