I have run across a bug using the Image Manager. When selecting an image from Add Image, there is no option shown to Apply the Image to the Image Files. As a result, the image is not saved – unless you tap the white bar at the upper right corner. I found this out by trial and error. Perhaps the text is missing or it is white text and not visible on the white bar.
I have a 3rd Generation 11” iPad M1 Pro with SW 15.3.1. I have MDP 2 with version 2.311.