Hi all,
I had this issue where the fader would set the speakers to a few ticks towards the rear vs the front. The dealer replaced the entire deck for me under warranty at around 20-25k miles. I had to push hard for it but Kia does have a pitstop or bulletin for this issue and from what i remember, it literally states a if this and this and this condition is present under these circumstances (not literally yours necessarily just the way it's written up) the solution = replace radio under warranty.
If what I described is your issue, under no circumstance should you let it go and unresolved (in my opinion).