The fact that this happens only on certain sites doesn't mean that it can't be a server related issue.
A server may have different and independent configurations for each subdomains, but posting here is not helpful only the hosting provider would be able to help in such case.

All i can say is that at PHP level the visitor IP address is accessed using the standard PHP global variable:

Code:

$_SERVER ['REMOTE_ADDR']

So if it's not altered by the server, even it sounds weird to me there could be some Joomla plugin installed altering that variable, but nobody can know.