This is version 12. It might be fixed in 13, but I can't check.

I have, indeed, created my own poses, but I've noticed the behavior described for all pose files, created by me or not.

It isn't that the pose won't apply (it does), the behavior is that applying a pose to a figure with IK on, which has many keyframes in the document, can take a long time, and can actually alter the already-created keyframes for that figure in the document.

As mentioned, I can use a completely blank pose file ( just open and close braces) and this behavior manifests. It seems like turning off IK on the target figure (if it's on) is a first step performed by Poser when applying a pz2 file in general. The observation that I can re-name the file to FC2 (an expression), and have poses apply with no problems seems to bear this out.
  That renamed FC2 file does require the edit as above, but that just seems to be because some library types will only read one top-level object whereas pose/character files will accept more than one.

Another way to see this problem:
   It seems that various conditions will cause IK application to calculate with greater error. One thing that causes this to a high degree is non-uniform scale of one of the IK segments, along the the "length" axis. For instance, load any poser figure, change the "yScale" of the thigh and shin to soemthing like 70%,  fix the foot position to be in reasonable range of the IK motion, then turn IK off and on a few times and watch what it does.
   Since pose application does this " IK off/on " each time, all existing keyframes of the pose get more and more messed up as every pose is applied form the library.