[...]Why is vortex ring not instantly solved as soon as auto is entered?[...]
Aeroelastics.
You get into VRA because the main Rotor blades pump too much air for the environment.(Same thing happens when you heat up oil and those little bosses occur - another story)
This happens mainly because the Rotor blade is elastic itself and steered by the inner part. So the inner part has an higher AOA for a short time which supports the outer vortex with the energy it needs. (Would be interesting if any K-Max pilot has gone into an VR)
Then you press the stick forward because the airstream around the tail boom is more directional. (As seen in the paper, backward movement increases the vortex in front which is seen by increasing angle in 4.2)
IAS will help you now by pumping the Vortex away.
When you simply pitch to auto you will gain no IAS.
And the Airflow has to cope with the new AOA firstly, as i said, from the inner (part of the)blade to the outer (part of the)blade. This does brake your rotor. After you got out of the Vortex you still have to cope with an high rate of descent, which you cant do just by pitch. It yust shortens your time to evade obstacles.
Greetings,