DNS record for help.example.com returns 404

I have a domain example.com. I'd like to map help.example.com to example.com. The DNS records are:

Hostname           Type     Value
example.com        A        93.184.216.34
help.example.com   A        93.184.216.34
www.example.com    CNAME    example.com

I can access the website with both example.com and www.example.com but when I try help.example.com I get 404 Not Found.:

Not Found
The requested URL / was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

So I'd like to know why the A record for help.example.com is not redirecting to example.com.

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Extra information: My .htaccess consists of the single line:

ErrorDocument 404 "Cannot find that URL"

If I try to access example.com/some_random_string I get (as expected) Cannot find that URL. However if I try to access help.example.com/some_random_string I get the above Not Found message. So it seems that .htaccess is ignored by help.example.com

I also tried to replace the A record with:

help.example.com   CNAME     example.com

But this returns the same Not Found message.