Hi @emiliano.lupo,
Welcome to the community! Feel free to Introduce yourself 
to the community if you want to! 
This is a limitation on Revit’s side actually and we are working on a workaround atm. Revit can’t create a single closed spline. So if you split it, join it back, create a planarsrf and send it, it should create the floor. 
We are aware this is not the best way for the end users and eventually we want to do this conversion automatically.