UPDATE: "By design"
This issue is caused by the particular image having excessive space (alpha) at the bottom area. When sprite packer packs it, that space is trimmed. And since our UI image never takes into account the mesh defined by the sprite packer, this kind of glitch will happen.

To fix it. Just set the "Mesh Type" of the sprite to "Full Rect". It defaulted to "Tight". This option can be accessed after setting texture type to "Advanced" in texture importer inspector.

Steps to reproduce:

1. Open attached project
2. Open scene "bug scene"
3. Run scene and notice that artifact appears below "btn1"

Note: This only happens when using Sliced or Tiled image type. Also happens on builds.