@fastian04 This is indeed very weird behavior, and is actually a make-or-break situation for me since I had almost decided to buy a 2.5 Sonata until I saw your post. I've found this being discussed on other forums as well. As per some of those discussions, the owner's manual says the following:
Pull the [+] or [-] paddle shifter once to
shift up or down one gear and the system
changes from automatic shift mode to
manual shift mode.
To change back to the automatic shift
mode from manual shift mode, do one of
the following:
• Gently depress the accelerator pedal
for more than 5 seconds.
• Drive the vehicle under 6 mph (10
km/h).
• Pull and hold the right side paddle
shifter.
Theoretically, this means the car will not shift to auto if you're not pressing the accelerator. Can you confirm if this is what you experienced? If this is the case, while it is still weird, it may be okay for driving downhill. Will definitely need to get used to it.