Adventurers form groups with others that fits roles they lack in their adventuring team, just because they have a good selection of adventurers in their respective roles, does not mean that they have knowledge about absolutely every facet of dungeon diving.
This is the exact reason for why you would have a Dungeon Sherpa, who's sole focus in dungeon diving is to take notes of anything that is even remotely interesting or worth noting down, to learn as much about the dungeon's different floors, from its monsters to its environment and its nature and ecology.
The adventuring group are aces, yes. But they've encountered a similar type of monster to this one, and just like how a person might know one type of mushroom or plant, the same person might not be aware of the other type of mushroom or plant that is nearly identical at first glance as the other.
Thinking that this monster was the same as all the others they have encountered makes sense, to a degree.
However, I do agree that they should have been a lot more cautious in their actions.
They are inside of an unfamiliar dungeon to them, they had to rely on the Sherpa to know about its different floors and enemies, and now they are on a floor that no one, not even their Sherpa, has previously set foot on.
They should have definitely taken all that into account and assumed that not everything may be as it appears, and that even the most normal and/or innocent of things might be dangerous or pose a threat.
There might even be things like new plants that could be dangerous, take real life mushrooms or plant, for instance, they may appear innocent at first glance, but touch or eat them, and you could quickly end up experiencing anything from minor pain and annoyance to even death.
This wasn't the smartest action on the group's part, definitely not, but it also isn't entirely unexpected.