Operating system: Win10P Pro 64 bit
Graphics card: MSI 1080 - i7 6800k 16GB ram
version: 2.80 (sub 37), branch: blender2.8, commit date: 2018-12-14 16:29, hash: c7cc97c7b90, type: Release
build date: 14/12/2018, 08:44
Short description of error
This seems to work OK in 2.79 so guess is a bug.
In 2.8 -
With the default cube make a change the material so it has two shaders (say red and green) and a mix shader.
Set the factor of the mix node to say 0.2 and set a key frame for it.
Now Alt-D the cube to make another.
Go to Object/Relations/Make Single User and select Object&Data&Materials to make it single user.
Now go to the Shader Editor for the new cube and find the Factor of the Mix Shader and set it to a new value.
The two cubes will be different colours.
Press 'i' with cursor over the Factor window to make a key for it. Both cubes will become the same colour.
This is I think because Blender believes the Mix Shader for the two different materials are the same mix shader.
Hope that makes sense.