System Information
Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce GTX 980/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 461.40

Blender Version
Broken: 2.91.2 + version: 2.92.0 Beta, branch: master, commit date: 2021-02-11 16:22, hash: rB314525b8cfae
Worked: ?

Short description of error
I have found that the Mix shader result misses Subsurface shading if shaders like BSDF Principled or Subsurface Scattering are connected through the second socket (Input[2]):

In this picture you see two identical shader connected to a Mix shader. There is a light object inside the cube to see the Subsurface effect.
Problem: Switching the factor value of the mix shader should have no visible change, but value "1" shows that Subsurface is gone.

Exact steps for others to reproduce the error