Most models of iPad require a Power Adapter rated at 12W or greater to reliably charge. For large-screened iPad models, if charging while using the iPad, particularly at maximum screen brightness as will often be required in the cockpit, an external 12W supply may be insufficient to maintain the current state of charge, let alone simultaneously re-charge the iPad battery.
Be aware that many USB ports, in particular those of a computer, commonly cannot source sufficient power to charge an iPad. Unless explicitly designed for charging connected devices, USB ports are typically limited to 0.5-1A @5V (i.e., 2.5-5.0W).
Your iPad Pro was supplied with a USB PD (Power Delivery) Power Adapter. These Power Adapters can supply power, and charge, at higher rates than standard USB connections will support.
By design, when charging from a standard USB supply (5V), the iPad will limit power transfer via its USB-C (or Lightning) connector to 12W (i.e., 2.4A @5V); when using a USB PD compliant power source, iPad can increase its power transfer to 18-20W (typically 2.0A @9V or 1.5A @12V).
It is highly likely that your cockpit (Strata) USB power supply does not support USB PD - and will therefore be operating at the standard USB bus voltage (5V). As such, regardless of the power supply power capability, the iPad will be limited to drawing 2.4A via USB - this being insufficient to both operate the iPad at maximum screen brightness (with running Apps) and maintain the battery charge. Consequently, to provide sufficient power to its electronics, power load is being shared by both the external power source and the iPad battery - resulting in the noticeable depletion of the iPad battery over time.
In addition, iPads thermal management may be playing its part in limiting the iPad’s overall power dissipation. High screen brightness, coupled with running Apps, will generate heat - which must be adequately dissipated or managed. iPads thermal management may impact the ability to charge. Newer models of iPad, in particular iPad Pro with larger and brighter screens, make heavy demands upon available power dissipation - which in domestic or “regular” environments are rarely challenged. Exceeding bright environments create challenges rarely seen by many users.
As a pilot myself, I am acutely aware of the many limitations of operating an iPad in the cockpit of GA aircraft. I hope this insight offers some useful explanation of the likely cause of observed behaviour.