ipad pro 13 keeps charging even though its at 100%

Your iPad will suffer no ill effects through being connected to its Power Adapter for extended periods. Your iPad will automatically regulate and optimise charging - and will not overcharge the battery.  The iPad will indicate that it is connected to external power at all times that adequate power is available.

If you have limited the battery charge to 80%, your iPad will occasionally charge to 100% - this being necessary to maintain the battery at peak efficiency and calibrate the charging process. This behaviour is normal.

When used in kiosk applications - where external power is connected for extended periods of time - the iPad may modify its charging strategy and limit battery charge to ~80% of maximum. This modified charging strategy is designed to reduce chemical ageing that might otherwise occur if 100% charge is maintained at all times. Nominal 100% charging will be restored when a normal charge/discharge routine is restored.

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

About iPad and iPhone Charge Management Feature – Apple Support

Why do you think it’s still charging?

If it is charged to 100% it will have stopped charging.