What's with the memory effect on the iPad screen when you switch from one app to another?

Question

iPad

I sent a letter from the Yandex-mail application, then switched to the native VKontakte application. Against the dark gray background of its menu, I saw a barely visible image of the keyboard and an image that looked a lot like a letter just sent. After a while it disappeared. I took a screenshot, but on it, these "watermarks" were not preserved, it is clear that some other layer was involved.
The question is banal, what was it?
iPad mini tablet, 9.3.5 axis