According to RFC 6122 a JID "username@my_domain" should be allowed.
The domainpart for every XMPP service MUST be a fully qualified
domain name, IPv4 address, IPv6 address, or
unqualified hostname (i.e., a text label that is resolvable on a
local network).
(as well as e.g. username@127.0.0.1)
So I think it's probably a bug in Openfire.