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Like others have said; It is not the OS that is the issue. It is the underpowered charger that comes with the iPad Pro. The first Gen iPad Pro has the same issue. You need to use Apple's 29 watt USB-C charger.
Apparently the Anker 30 watt linked above will fast-charge as well.
Don't forget, you also need a USB-C to Lightning cable too.

It is probably both the underpowered charger, known issue since the original 12.9 and the beta, known battery killer.
 
Like others have said; It is not the OS that is the issue. It is the underpowered charger that comes with the iPad Pro.

In the end, your statement might be 100% correct, but that cannot be applied with certainty since he decided to run a very early beta OS right out of the box.

Don’t get me wrong: we all know the charger should be bigger. But there are other variables at play here too.
 
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