I haven’t played much dnd, do people expect stat blocks to be fixed? Like, is meta knowledge such a part of the game that its a dick move for gm to change it?

Meta kills some kinds of fun and, for lack of a better term, can be like topping from the bottom. If you’re trying to keep it out of your game with this method, your group is dysfunctional either because there are misaligned goals or the gm and players are in a pissing contest.

Im not really asking from a power-dynamics standpoint, more as a narrative tool. As a gm I change the world to make what I see as a better story (no cheating necessary I’m literally the universe).

Like, if the players do an ambush of BBE and (according to the rules) one-shot ‘em, he’d have a shielding amulet leaving him heavily maimed but alive. Not cause how dare you kill my beautiful npc, it’s just kinda anticlimactic otherwise.

(Unless the ambush was a huge challenge in itself. Then we’re good, murder away)

from my experience i’d say players expect that certain monsters behave a certain way,

e.g. trolls regenerate, displacer beasts teleport around and a vampire can suck your blood, rather than a bear has 16 strength 60hp and so on and players value consistency, so if a kind of monster had no fire resistance the first time they’ve encountered it they should not have it next time only because the wizzard destroyed 5 of them with a fireball.

some players read published adventures though and get irritated if a game master changes stuff, but those guys can run their own game where everything is straight from the book rather than adapted to the group.