Air flowing over the open roof creates a suction. It sucks the air through the fresh air intake and out the vents. The blower is not running. Of course, closing the vents prevents the airflow. This is not a "Hyundai" thing. All the cars that I have had with moon/sun roofs have done this.
Can verify this, 2017 Limited is the fifth car with a sunroof, or is this a moonroof, can never get this straight.
Let's learn what the owner manual says on this subject.
"Automatic Ventilation
(if equipped)
When the ignition switch is in the ON
position or when the engine is running
and temperature is below 59°F
(15°C) with the recirculated air position
selected more than five minutes,
the air intake position will automatically
change to the outside (fresh) air
position.
To cancel or reset the Automatic
Ventilation
When the air conditioning system is
on, select Face Level mode and
while pressing the A/C button, press
the recirculated air position button
five times within three seconds.
Sunroof Inside Air
Recirculation (if equipped)
When the heater or air conditioning
system is on with the sunroof
opened, the outside (fresh) air position
will be automatically selected. At
this time, if you press the recirculated
air position button, the recirculated
air position will be selected but will
change back to the outside (fresh) air
position after 3 minutes.
When the sunroof is closed, the air
intake position will return to the original
position that was selected."
So resetting of the MVAC controls when the sunroof is opened is normal.