There appears to be a bug, introduced in iPadOS15 and carried forward to iPadOS16, that impacts markup/cropping of screenshots and Photos. Until the underlying issue is resolved, there are reliable workarounds for this issue.
A usable workaround, that works for many, is to first save the cropped and/or marked-up image - at which point, when reviewed, the markup appears to have been lost. Saving another screenshot/image will make edits made to the previously saved image visible.
If you have an Apple Pencil, triggering the screenshot using the associated corner swipe gesture will cause the screenshot to be saved with markup edits - without the need to take a second screenshot. The corner-swipe gestures are configured in iPad settings:
Settings > Apple Pencil > [Pencil Gestures] Left | Right Corner Swipe - select Screenshot, Quick Note or Off as required
Anyone that is experiencing issues with screenshot markup are strongly encouraged to submit a bug report, to Apple, using Apple’s Product Feedback portal:
Feedback - iPad - Apple
Every report submitted counts; until sufficient reports have been received, Apple may not assign resources for the issue(s) to be investigated and corrected.
When submitting a report, be sure to include details of both the iPad model and installed version of iPadOS in the body of the report; do not rely upon the associated pick-list, as this is rarely up to date.