Yes, this looks to be a problem on the (Hummingbird) (r)lpd(aemon)(1M) side.
To be sure, (temporarily) set up a remote printer from the HP-UX system to itself.
Are you using rlp(1M) directly or, as is the normal case, from the /usr/lib/lp/model/rmodel script? In both cases, how do you invoke rlp(1M) c.q. lp(1), i.e. which options and parameters?