Has Rubio ever held Cuban citizenship as a result of the workings of Cuban citizenship law? If he has, then there is doubt. If he hasn’t, it’s a question that can be settled since he would be under the sole jurisdiction of the United States, and I assume there are no other nations making a citizenship claim upon him. It all comes down to jurisdiction.
http://powderedwigsociety.com/eligibility-of-cruz-and-rubio/#
Pretty straightforward and easy to understand presentation about Cruz and Rubio’s eligibility to run for POTUS.