Am I correct in understanding that you would strongly object to a GM changing an NPC's actual statblock behind the curtain for any reason, but you have no issue with a GM showing the players a phony NPC statblock in order to preserve a mystery? You would consider the former to be a breach of trust but the later not to be?
I'm more-or-less the opposite. I have no issue with a GM changing unestablished-in-play elements of an NPC's statblock behind the curtain, but if the GM is ostensibly pulling back the curtain by showing me the statblock, I would consider it a serious breach of trust if the shown statblock was inaccurate.
That's a good question. I think it all depends on the game and the table expectations.
In the scenario as described, there was clearly nothing more to the girl, and there was no compelling reason for the GM to not share that information as indicative of what the characters could derive from the scene. So it was shared. The players were informed and they based their decisions on how to engage with the encounter accordingly.
If there's information beyond what's obvious in the scene, and the discovery of that information or not could impact how things play out, then I'd probably account for that. I'd call for checks or if I shared a stat block I'd leave certain spots blank, or only share the relevant stats, etc. Does this cue the players that there's something more there? Yes, very likely. Do I care? Nope, because without knowing what it is, how can they act on it?
The hesitation for many seems to revolve around letting the players in on a little bit of something even if their characters wouldn't be aware of it. I don't really get that. I expect my players to handle that in a way that seems fit to them, and I don't really worry about it.
Now, as for changing stats "behind the curtain"....I don't know. It depends on the reason for doing so. As far as I'm aware, there's no rule that says you should do that. There are general bits of advice that could be interpreted as supporting such a decision, but I don't think most of the time that folks say "You should take the stats of a monster and change them mid fight if things are too tough or too easy". So I'd have to have a really compelling reason to do so.
It's hard to not see the reason as being to Force an outcome. Like, "this may be a TPK, I better adjust the monster" seems kind of pointless and doesn't seem to be letting things play out as they may.
I don't know if I'd say I'm dead set against it, but I think there has to be a really compelling reason to do it, and my threshold for compelling is probably pretty high.