Yes. So they appear or disappear out of nowhere--just out of sight. Exactly as video games do. Almost always with a planned possible combat encounter!
For goodness' sake, are you presuming that anyone who doesn't do things identically to how you do just instantaneously manifests things directly in the players' line of sight? Like are you seriously trying to claim that just because others approach things differently, they must be doing the most aggressively anti-realistic methods possible?
This is precisely what I mean when I talk about double standards being used all over the place in this thread. If you wouldn't do something like that, why would you immediately presume, most uncharitably, that others instantly WOULD do so? Especially without ever asking to clarify first!
The point was that your description applied to both things, so I didn't (and still don't) see what the distinction is.
Really? So if the party failed, say, a stealth check early on--not badly enough to give the game away, perhaps, but enough that they've now left visible signs of entry--you wouldn't have the guards respond by beefing up security a bit?
Again, it's inherently unfair and uncharitable to presume that others "spawn in" monsters pop pop pop directly in view. So what's the difference between "failed stealth check -> more of the guards that we could pretty clearly tell already existed are now assigned to patrol the halls" and, y'know, any of the stuff you're rejecting here? Because if we assume the GM doesn't just literally pop things into existence on a whim, and only introduces or removes things that have reasonable fiction behind them...then, as stated, I do not see any difference between the things you're describing and the things you're claiming you oppose.
But...the former thing and the latter thing are identical in both of these sentences. You are changing the outcome, that's...literally what adding or removing forces does? The exception is a hole the same size as the rule itself!
And by adding or removing things from a combat, you are, necessarily, changing things to direct the flow of the game--toward something you consider challenging but not overwhelming, and away from something you consider overwhelming (or, presumably, underwhelming--if you felt the existing mechanics would be inappropriately dull/inadequate, when compared to the information available to the players).
You're saying you're not doing a thing, only to then give a more specific example of...doing that exact thing. You just require that doing so begins and ends with the fiction: it must arise from what is known, or what could be compatible with what is known (e.g., the party probably doesn't know the exact number of bears in this forest, or the exact number of guards in this palace, or...etc.), and then whatever results come out the other side must flow back into the world, with the appropriate consequences.
That's very literally what the rules for Dungeon World (and other PbtA games) explicitly require. So, again, what is the difference? Dungeon World doesn't permit "spawning" enemies in the way you've described any more than you do. In fact, doing so is explicitly against the rules, and the GM is supposed to follow the rules!