I'm not sure what would cause this, but my guess is that something was being set incorrectly or somehow got reset. Which IK solver are you using? It is 3D or 2D?
I know at one point I was getting all sorts of issues with the FABRIK solver sometimes resetting the bone data on itself in the editor, which I don't remember how I fixed or what was causing it, but it might be something like that going on here.
Also: Technically no, a tip bone is not necessary but it will make the results easier to calculate, which in turn tend to make IK look better.