Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 1060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 451.48
Broken: version: 2.92.0, branch: master, commit date: 2021-02-24 16:25, hash: rB02948a2cab44
Worked: (newest version of Blender that worked as expected)
Short description of error
Selecting a rig from the drop down is broken. It only works if you use the eyedropper to pick the rig
Exact steps for others to reproduce the error
Link a rig into a scene.
Make it animatable by using a library override.
Create a copy transforms (or any other kind of constraint) either from a Null or from one controller to another within the rig.
Use the drop down menu to pick the rig as the target, then pick the desired bone.
The location will update, but it won't animate/follow edits.
Go back to the constraint and clear the rig, then pick the rig via the eyedropper, then the desired bone. Voila it works now!