I had this problem a few nights ago in the Urban Combat fractal. One of my guild/party members was complaining over Mumble that he was invisible, and then that his character kept changing appearance. Before long, I noticed that mine was also changing.
I was a (human) guardian, but my charr avatar started off in heavy armor, then swapped to something that looked like leather, and then to just shorts of some kind. I suspect this was a heavy/medium/light armor thing, but I was wearing heavy armor the whole time. It eventually cycled back to heavy, but I took a couple of screen shots. I missed the “medium” model, but maybe this will be enough to help.
As I recall, it eventually stopped switching… Or maybe I just stopped noticing.
“It is the stupidest children who are the most childish
and the stupidest grown-ups who are the most grown-up.”
- C. S. Lewis