Is it a glitch? Not all followers can be stewards, maybe not all housecarls can be.
Why not?
You don't think it's a just a little peculiar that
every housecarl can be appointed as a steward
except for Argis?
Also, why do you seem to automatically go on Bethesda's defense whenever someone throws out the possibility that something
could be a glitch? Do you read sentences like "I think this might be a glitch." as "This game is an utter travesty which I am compelled to heap insults and derision upon, and everyone involved in producing it should be rounded up and marched off a cliff."