everything in solb is supposed to see invis, the reason is because solb was supposed to be the main high level dungeon before kunark and they wanted it to be very difficult
everything is correct on this server in regards to this
http://bcope1.www6.50megs.com/monk/guides/solb.html
"All monsters can see invisible, so corpse recovery can be a pain, plus you can't easily get to your destination without killing everything on the way."