We have seen many reports of iPad Air4 battery charging being apparently limited to approximately 80% of full capacity in some circumstances.
Whilst unlikely, perhaps you have a problem with your Power Adapter. Start here:
If your iPad won't charge – Apple Support
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.
Other reports seen within this Support Community suggest that temperature may be a contributory factor; the iPad is designed to reduce the charging rate if the temperature is too high for optimal charging. Whilst purely speculative, it is possible that the recent iPadOS update has erroneously changed the temperature calibration for your iPad. If so, a future update may correct this irriation.
More information about iPad’s battery management can be found here:
About iPad and iPhone Charge Management Feature – Apple Support
Recent changes to iPadOS may have altered the charging strategy of your iPad. Speaking of my own experience, the iPad Pro from where I make this reply has recently changed its own charging strategy - and now rapidly charges to 100% capacity without reducing the charge rate, or pausing charging, at ~80% of maximum.
