The reason why I'm coming to this conclusion is because I have two followers in another hearth. They are two housecarls. The housecarl from Whiterun became the steward while the Falkreath housecarl resides there as well. Both of them, including the steward, has the option for you to ask them to follow you when you talk to them. On the other hand, Illia, does not show the option for me to ask her to follow me after I had made her my steward at Windstad.
Are stewards supposed to no longer be able to follow you after you have made the follower your steward? Otherwise, I'm thinking particular followers that become stewards are glitched.