I changed question alias, new url but old url still online. How come?

Good morning.

I changed a question's alias, so to have another url. I also set a 301 redirect from the old url to the new one.

However, I noticed that the old url, to the same question, is still online and does not expire and therefore it doesn't redirect.

If I change the alias to the question, how come the old url with the old alias is still online?

thanks