Is it abnormal to have knee numbness? Yes. The medical term is neuropathy, kind of a catch-all for conditions related to diminished sensation. You should have sensation in your knees, but for whatever reason, it isn't getting through.

You say you notice the numbness at the end of your ride. I'm guessing you never dismount until you finish the ride. I'm wondering if your numbness sets in long before you finish the ride and become aware of it. What if you tried dismounting much earlier, say before halfway? You might be able to figure out exactly how long (and/or how much effort) it takes for symptoms to appear.

As to what's going on, anyone's guess. You can try typical conservative management - anti-inflammatories (creams like Voltaren applied directly to the skin of the knees, or Tylenol / Advil / Aleve etc taken orally), rest, ice, elevation, compression, etc., and see if they have any impact.

If not, best to have this checked out by a physician, if for no other reason to rule out other conditions.