It’s so stupid that you can’t transmog ONLY the artifact offhand without also transmogging your main hand.
This would be so trivial to fix. Just make Level 1 items with no stats on them that have the appearance and mail them to anyone who’s unlocked those artifact appearances. Problem solved. You don’t even have to go in and fix the buggy artifact weapon code.
Do it now. Getting sick of not being able to transmog the Worldbreaker shield appearance.