Why My new iPad Pro charged only 80% ?

Your iPad will automatically limit charging to ~80% is some circumstances to protect the battery - this often occurring when the ambient or device temperature is relatively high.

Do you leave your iPad connected to its Power Adapter for extended periods? If so, this is not of itself an issue - as the iPad will optimally manage battery charging. When connected to a Power Adapter for an extended period of time, such as when used in Kiosk or PoS (Point if Sale) applications, iPad will change its charge management strategy - and reduce the maximum battery charge level to about 80% of maximum capacity. 

This strategy is intended to extend the life of the battery by reducing chemical ageing that occurs when the battery is continuously maintained 100% charge. As soon as a “normal” charge/discharge pattern is resumed, charge to full capacity is restored. If your iPad has paused charging at ~80%, simply disconnecting the Power Adapter from the iPad for several seconds may trigger completion of the charge. 

Temperature is also a contributory factor in charging being paused; the iPad is designed to reduce the charging rate if the temperature is too high for optimal charging. Summer high temperatures may cause this protective measure to be triggered. Try moving yout iPad to a cooler location whilst charging.

More information about iPad’s battery management can be found here:

About iPad and iPhone Charge Management Feature – Apple Support

See the answer marked as "Apple recommended" in this similar thread:--> Ipad Air battery stuck at 80% - Apple Community

Regards

Giulio