This has been a known issue reported since HW. I know that it was an issue I encountered a lot during SB, usually when trying to weave in Sprint between my GCDs. For some reason, it would just randomly break the physics of the clothes—which is far more noticeable than some realize when you’re wearing a long dress/robe/skirt. I think it has something to do with the fact that ASTs have motion physics for their gear when their weapon is drawn (i.e., it flutters ethereally), and something happens to just break them completely during combat. I’ve never had this issue on any other job; and the only difference between AST and other jobs are that their gear animates when their weapon is drawn.