The delve Shipwreck Shoals on High Isle definitely has a bug with the final boss, Captain Ithala. It's happened to me on at least five separate occasions. I see that others have posted reports about not getting the 'delve completed' achievement and I believe that is due to this bug. I can provide more specific details.
She spawns, with a reef viper.
Then it always happens WITH A PARTICULAR BIT OF DIALOGUE.
She says: "What a nice bunch of plump guppies to feed my pets" and then she disappears -- not killed, just disappears (which is why the delve completed isn't working; no plunder skull during Witches' festival, for example).
On the last occasion, I waited for her to respawn and killed her without difficulty -- that time, she didn't say that bit of dialogue (instead, she said: "My vipers will tear you to pieces").
The bugged experience began with the dialogue "The fools getting stuck in these shallows are easy pickings", though she doesn't disappear until the "guppies" line.
The not-bugged experience began with the dialogue "Gather more of these documents. They'll help the Fleet Queen guide our ships to plunder."