Operating system: Windows 7 64
Graphics card: nVidia GeForce GTX 970
Broken: 2.80.72, e1d1899e7290, 2019-5-25
Worked: 2.79b official release
Short description of error
In 2.79b you can have duplicates of linked groups with proxy armatures as long as they were in separate scenes. This could be done either by creating a new scene and linking the group again or by copying a scene containing the data.
In 2.8 all instances of linked collections are shared and only one proxy armature effects the linked collection regardless of the data being in separate scenes.
Exact steps for others to reproduce the error
The attached file "Proxy System Regression - Source File" contains the linked data used in the other attatched file "Proxy System Regression - Linked Data File".
Opening the latter file in 2.79b, each of the two scenes has the group instanced and a proxy armature deforming it in a different way. In 2.8 however, only one deformation from one proxy armature takes place across any scene. You can also attempt to link the group/collection in a new file and add a proxy armature but the result when attempting to create a duplicate scene or link the data again will work as previously described.