I'd wager they're just very shy and socially awkward. A lot of people in fandom are very socially awkward in real life, and socially awkward people can forget about manners and be very quiet and withdrawn even while they're enjoying themselves a lot. I think odds are pretty good they didn't realize they were being rude at all - you might simply want to cautiously bring it up and see if they try to change.

But I get you, anon. There are two fandom friends I've visited in real life and stayed with several times, and it's kind of painfully noticeable that one of them is the nicest, most generous and generally fun-to-be-around host in the world while the other... isn't. I still love her, but sometimes she just does friendship weirdly and it hurts. I know it's just awkward social skills and she actually is really excited to hang out with me, but it still bums me out.