That does sound like Alexia/Andras NTR scene, but the description is so vague I can't be certain. Please provide more details.
I'm pretty sure he talks about the scene in 'the_challenge' event (alexia_corruption2.rpy)
Is this a joke? The game asks - do you want to see NTR scenes? You say no, but they're still there. (special hello to the box scene).
When she opens the box and masturbates, there are options. Surrender or resist. After four or five resistances, the answer is give in. All five answers.
I mean, she's being fucked.
To put it simply: you saw it because you probably already let Alexia fool around with Andras. And turned NTR off in the middle of the game. And ignored hundreds of possibilities to
not see that scene (like already pointed out).
A bit longer answer:
Event 'the_challenge' that includes the scene you described requires that the game variable
alexiaAndrasObedient
has already been set to true. That happens at the end of event 'powerless' (where Andras punishes Drokk). That's the true 'no turning back' point on Alexia/Andras NTR route.
Technically: yes, there're no
NTR == False
(or similar) checks during those events. But you can only see the event 'powerless' if Andras' influence on Alexia is already 6 or higher (among other checks). That's the variable you pump up
only when you let Andras to "pump" Alexia.
And there're NTR-checks before you can do that. I'm not 100% sure if
every +x to Andras' influence on Alexia involves sex, but that's the general rule. And it's highly unlikely that you saw 'the_challenge' event without letting Andras fuck Alexia long before that. And several times.
e: In summary: I wouldn't consider it a bug.