As venitia (= king tier), I vassalised myself to byzantium to blob inside it. (I needed to lower their crown authority BTW

)
100 years later, I decide to become independant. I make a faction, and two dukes joins me. I launch the revolt, win (not really due to my allies/ temporary vassals) and become really independant. Of the two allied dukes, one (a neighbour of mine) is now my permanent vassal and the other is independant (this one is an esatern duke)
Why did i get one of them as a vassal ? Is it intended to wrok like this ? Is it a bug ?
If it is a bug, I guesse the fact that the duke who is my vassal became duke during the war has a link with it. (His father joined my faction, we started war, he died on a battle, we win the war, the new duke is my vassal)