Ok, this is starting to get ridiculous. I just joined 5 times in a row the final lfr part at Lei Shen - although I already killed him this week.
I don't mind joining a begun instance, usually it will save you the trash and in the case of Lei Shen you also might have some determination buffs.
But when I have killed him once in the week, and queue again, then I want a fresh instance. It worked perfectly before - as soon as you have a ID I always joined a fresh one. I have no interest at all in wiping again several times on a boss I won't get a chance for loot.
And if you leave (instantly), you get the dungeon deserter debuff. Even if you alt-f4 and wait five minutes you will get the deserter buff now. So your best bet is to port out of the instance and ask for a kick, but sometimes even that takes up to 15 minutes because the other players don't want to or cannot kick someone any longer.
So seriously, fix it, waiting up to 45 minutes to join a pointless dungeon group is not fun for anyone.