What is it with the confusion?
OP said he/she reported to his/her school when status changed from F1 to LPR. And now OP just recently became a USC and didn’t state if he/she was still in school or not.
Also, CBP wouldn’t have let OP back in had OP claimed USC when in fact not, even with a valid LPR card.
The question was if this was normal to be always asked if you were a prior F1 visa holder.
And answered, albeit, not directly but more of a suggestion.