Get more NX till May 14th when charging codes

It goes to prepaid

False. It goes to whichever medium you got it through.

I charged 10k credit and 25k prepaid on two different accounts today (Maple and DFO, if you need to know), and I got 11k NX Credit and 28k NX Prepaid.

So how can they tell whether you spent the bonus or the original sum?

Amount left. Doubt is not on your side. I believe you spend "older NX" first, so unless you spend it all or charge NX after the event, you can lose it.

Remember that charged NX expires after one year. I think they implement the same system for this.