Subdomains have a life of their own and occasionally do or more often don't do what you expect. Software in generally doesn't like "unexpected"
I don't use subdomains in my forums but I think its a matter of preference and not whether or not it will work. Oops, I lied, I do have a demo and test system in subdomains that seem to work so I see no reason why it shouldn't.
HTH, Michael