seki Thank you for reporting the issue. Using the project you provided earlier, I was able to reproduce the issue you mentioned, but I have not yet found a consistent way to reproduce it (i.e., a way to reproduce the issue when starting from a new project). I was unable to reproduce the issue by creating a new project, adding two bones, setting an IK constraint, adding a few constraint keys during animation, and exporting in 4.2 format. Therefore, I believe there may be another underlying factor, which I am currently investigating. I will let you know as soon as we find out something.
