It could be due to another cause which wouldn't be covered by a warranty. However, if it's the same problem, then regardless of whether it would be covered by another warranty, it would be covered by the basic bumper to bumper warranty. This follows because the fix -- per protocol -- is to replace both nuts. Assuming the problem has returned, the warranty would apply since the repair is necessitated by a faulty initial warranty repair.
+1.It's the tension washer between the axle and hub. The nuts are usually changed at the same time, but the washer is the main cause. Full thread with details linked below.