Assuming they are using a traditional bechamel base to make the cheese sauce, then it seems like something as simple as not warming the milk up before mixing it with the roux. Cold or room temp milk can kill a cheese sauce pretty quickly.
And then, are they using pre-shredded bagged cheese? Or are they shredding from a block? I've found my best cheese sauces come from freshly shredded cheese and bagged cheese is more prone to a grainy mouth feel.
Lastly, are they adding that cheese a little at a time? Allowing the cheese to melt and the sauce to come back to temp before adding more makes a big difference.
In a fast food setting, there is so much that can go wrong with a homemade cheese sauce because it can be a finicky product to make if you're not careful
I post a lot using my phone. Most "typos" are probably a result of autocorrect