This an issue with the root bone of an IK chain when a pole target is used. IK panel locks and limits on the other bones should work fine (to a certain extent at least). See all the way at the bottom of this page in new manual (copy from earlier versions of wiki):
I don’t know if limits should be going wonky on the root bone too but they do as you have seen. That’s the way it’s been forever. It not being mentioned in the wiki means nothing. I figure that’s how it works and not a bug (some gnarly math goes on behind that IK stuff). Or it is a bug on todo list and/or waiting for armature code upgrade. It may just be something nobody bothered to report. File a bug report and you’ll find out for sure.
Hips (if that’s what these are) are usually not part of the IK legs in the typical Blender biped character rig. Usually the “hips” are one control bone on bottom of spine which both upper legs are parented (not connected) to. This IK root bone issue may be one reason why that is so.
There is an alternative to the pole target. A control bone at top of upper leg which turns the leg. Or simply rotating the upper IK leg bone itself. That’s no help in this case. Not with the hips being part of the IK chain.
Edit: Stiffness… limits… maybe I’m mixed up here. I know better than to post after only scanning the thread but I did. I hope this post helps anyway.