System Information
Operating system: Linux-5.4.0-51-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GT 630/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 390.138

Blender Version
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-10-13 22:29, hash: rB2cc7be3e4907
Worked: (newest version of Blender that worked as expected)

Short description of error
Datablock pin not working for bone constrains

Exact steps for others to reproduce the error
Add two armatures.
Select a bone in the first one and pin the datablock.
Select the second armature and see how blender ignores pinning when switching to posemode