Same problem here, I don't think I've ever seen the option to force PU unlocked.
If you have a PU, and the junior partner goes independent, you can then fight a "unification war" in which the force PU option is available.
But, it seems, it only stays unlocked as long as the CB is valid. That is, if the CB expires while you are still fighting, you no longer have it available (or indeed, any CB).