It probably doesn't help that they've also shown a tendency to split the party a bit during the initial foray into the dungeon....and the meat of the party is definitely melee-focused.
This is pretty much exactly what I've been experiencing in my recent runs as DM. Not long ago, my party decided to split the party. That resulted in a TPK (We were running the Hoard of the Dragon Queen, and they were killed by the kobolds in that cave part of the module. So much for that campaign).
Then, they act decidedly against character. In my current campaign, a little while ago they find a
Necromatic Book of Pure Evil. One of my players is a Witch Hunter. Another is a Rogue. The Rogue decides to steal the book while nobody's looking. The Witch Hunter catches him doing it. So what's the Witch Hunter do? Nothing.
Just wait, it gets better.
They lose the Book of Pure Evil, and begin the quest to find and destroy it. So last week they finally find it. It's locked in a room in a temple. On the door are warding sigils. They enter the room, and find it's a repository FULL of pure evil artifacts. So the conversation went like this:
Rogue: What's in the room?
DM: A whole lot. Idols. Phylacteries. Books (including the book you're looking for). Paintings. More idols. Pretty much everything.
Rogue: Weapons?
DM: Yes. There is even, prominently displayed, a very black and very evil-looking Sword of Pure Evil.
Rogue: Huh. And it's just sitting there?
DM: Yes.
Rogue: Prominently displayed?
DM: Yes.
Then the party gets ambushed, and there's a fight. After the fight, the conversation continues.
Witch Hunter: I think you should take the sword.
Rogue: I was thinking about it.
Witch Hunter: I know you were. You should take it.
Rogue: It might be trapped. It's prominently displayed.
DM: It's evil.
Witch Hunter: You're a Rogue. Check for traps.
Rogue: Good idea. I check for traps.
DM: It's pure evil. It isn't trapped. It is the trap.
Rogue: So not trapped. Excellent.
Witch Hunter: Take it. Just to see what happens. You know. Because.
Rogue: Good idea. I take the
Pure Black Sword of Pure Evil.
DM: I am not allowing you to take any further levels in the Witch Hunter class. Not until you learn how to hunt witches.
Witch Hunter: Such bull****.
DM: Being a Witch Hunter implies that you not only hunt the Witches themselves, but also their artifacts. Seriously. Think about it.
Rogue: I have an
Evil Black Sword of Vile Nasty. What good things happen to me??
DM: I give up. We'll meet again next week.
Combine all of this with the fact that the party never, EVER, runs from a fight, even if it's obvious that they're going to lose, and I, as the DM, get to figure out whether or not I want to TPK the party for being retarded. Again.
Mind, these players have been playing this game for going on 20 years. It's not like it's new to them.