Answer credited to @Asafmen
Focus on what they say;
After one division, half of the 15N genes were kept, in addition to that, half are “new” from 14N.
That is an indication to observe that in every division, in order to replicate its genes, the bacteria needs environmental Nitrogen for half of its DNA.
So, after another division, half will be the original Hybrid (since all of the previous generation was 100% hybrid) and another half newly formed DNA from environmental Nitrogen.