Exactly the same issue I am currently having now on a 2021 KIA Stinger GT V6 AWD I just purchased brand new on Feb. 11,2021. It started on the first week I had it and the gas gauge shows 3/4 full with about 130miles left to empty. then it died on the freeway because it ran out of gas. UNBELIEVABLE !!! I have been driving in the U.S. since 1975 and never ever run out of gas in any of the many vehicles I have owned and driven. So I brought it in at Concord KIA where I bought it and it stayed there for a week, and they have me pick it up and said everything was fixed. Yah, right! It was still doing the same thing after I picked it up and believe it or not, I ran out of gas again !!! RIDICULOUS!!! So it is back with the dealership again for the third time and it has been about 3-weeks now and they finally have a KIA technician from their So. California office come over and diagnose it and they decided to order the whole instrument cluster, which is according to the KIA Motors Consumer Center: "The FTS has authorized the dealership to replace the cluster as the FTS suspects there is an internal failure within the cluster".
At this point, invoking the Californian Lemon Law will be my next course of action, since KIA does not even have an ETA of the part.
Hence, I had it since Feb. 11, over 9-weeks now, but over 5-weeks of that is with the dealership.