Contrary to the popular belief, operating systems should always try to maximize the system memory (RAM) usage. If only half of the available memory is always free, why not to buy a cheaper device with less memory? The monitor App that you are using, whilst perhaps giving correct information, is only telling half the story.
Keeping recently used apps in memory (RAM), iOS/iPadOS can relaunch the App immediately from a saved state. Safari can, for instance, keep the recently accessed websites for the same reason. iPad memory management will release RAM immediately when some app requires more memory, so it basically behaves as if it was free and is immediately available for reuse.
In general, it is always better to let the operating system decide how to optimally manage its resources. Loading from a saved-state reduces reduces power consumption and response times - as Apps must otherwise be reloaded and reintialised.
In the event of a malfunction, a forced restart will fully release system resources during restart.
I note that your iPad is running iPadOS 13.6. You should update to the current supported version - iPadOS 14.6.