Atlas packing wrong when two mesh attachments map to a single input image

[HaraldCsaszar]

As reported in this email:
FMfcgxwDqTkCzQbTXMlvpvsZlDKGcxfg

[Edit:] Also reported by a different user on this forum thread:
http://esotericsoftware.com/forum/Only-first-attachment-will-be-exported-12435

Reproduction asset included in the first email as zip package.

Context:
When taking a look at the input image at Bugged Slingshot/images/Slingshot_idle_Top.png (the only colored one in the image set), it contains the complete V part of the slingshot.
In the Spine project, the mesh attachments Slingshot_TopFar2 and Slingshot_TopNear2 map to only the left or right half.

Reproduction Steps:

1. Open the included Spine project in the package.
2. Export the skeleton with setting Texture Atlas Pack enabled, and packing type Attachments, Texture Packer settings Packing set to Rectangles or Polygons.
3. Take a look at the generated png atlas file, a half slingshot is included in the atlas page, but only one half - the second half is missing.

Polygonal packing:

Rectangular Packing:

[HaraldCsaszar]

A different user also reported the same problem on this forum thread:
http://esotericsoftware.com/forum/Only-first-attachment-will-be-exported-12435

[NathanSweet]

3.8.61