ID 572258 - Merging an instance mirrored mesh results in broken UVs when using the Pack tool

Problem summary
Merging an instance mirrored mesh results in broken UVs when using the Pack tool.
 
Customer reported version
Modo 17.0v1
 
Customer reported platform
Windows n/a
 
Steps to reproduce
 
1) Download attached file and open it in Modo. 

2) Select the Meshe Instance in the Scenes Viewport, then use RMB > Change Type > Mesh.
 
3) Select both meshes in the Scenes Viewport, then use RMB > Merge Meshes.
 
4) Enable Polygon Selection and select all the faces on both meshes.
 
5) Navigate to the UV Layout, set the UV: to Texture and select the "Pack..." tool and use the default settings.
 
Expected behavior
UVs should remain intact after packing.

 
Actual behavior
UVs do not remain intact after packing.

 
Workaround
Unknown.
 
Reproduced by Support in:
Modo 17.0v1 - Windows 11, macOS 11.7 Big Sur - regression
 
Unable to reproduce bug in:
Modo 16.1v8 - Windows 11, macOS 11.7 Big Sur
 
Earliest version tested
Modo 16.1v8 - This issue doesn't appear in this version and has regressed.