I haven't seen anything that says anything about the penis feeling numb any time much less upon ejaculation. Are you using any of those creams or sprays that prolong the erection? They would make the penis feel numb.
The most common cause for numbness of any kind is a nerve issue. There is an article here from healthline.com that explains numbness and tingling and some of the causes. Is it possible that you are standing or lying in such a way that you are impinging on a nerve?
Online sites are wonderful places to get information so you can ask all of the necessary questions when you see your doctor, however, online sites can not take the place of your doctor's sound medical advice. If this is something that concerns you, you should see your doctor to rule out any serious issues.
Take care and stay in touch and let us know how you are doing!
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