Hi. I'm on the latest production build of Houdini, 17.5.229. I followed the rigging series by Michael Goldfarb and on part 3 - Arms, minute 34 there seems to be known bug regarding the constraints blend when making the IK/FK switch. The bug is stated as being fixed but it's still there as others have pointed the same issue.

Basically the IK, after that blend constraints is made and set to 0, by translating it rotates the L_arm_bone_2 (forearm), not acting like an IK is supposed to. And also the FK is not working anymore, even if the blend is put back to 1 (switching to IK).

Any solutions on that are appreciated. Thank you!

Found the solution.
Needed to add two blends for the constraintblend node. First blend 0, second 1 - FK is active, First blend 1, second 0 - IK moves. This needs to be for both arm bones.