bobster speculates that 5ar type-II is a unique enzyme in the body that never gets reproduced, in spite of evidence to the contrary. It's possible, but he is making a spectacular claim, so he needs the evidence.
I think a more plausible scenario is that it's simply homeostasis at work. If finasteride suppressed DHT for a very long time, that will be the new equilibrium for the body for some people even after quitting finasteride. It's analogous to how people stay fat after gaining weight sometimes, even if they go back to eating normally. Homeostasis is a powerful thing. In those cases, those people will need to jump through hoops to re-activate 5ar activity.