In my case the battery was draining very fast without any usage! Diagnostic test from Apple Support showed problem with compass sensor, peripheral lighting and barometer. Also the automatic screen brightness did not work and I thought that this model does not have this setting...
The tech of support told me that it is a hardware problem and that I should definitely take it to the service. Unfortunately, there isn’t any Apple Store in my country, Greece... I sent it to the authorized service center of Apple and they replaced it with a new one, without its box. I believe that it’s a brand new iPad because the first letter of the model number is “M”.
My new iPad Air 4 works perfectly! Although at first I thought it was software issue, it turned out to be hardware.
Call Apple customer support again and ask them to do a diagnostic test.
It is not normal for the battery to run out of 15% without use, and of course it is not a problem of the applications. It is our right to use our iPad with all its functions active, without thinking that its battery will be drained.
Ask Apple to replace the device. The guarantee is valid and Apple knows that there is a defective batch.
I regret that I kept the defective iPad for three months thinking it was a sotware issue!