There appears to be a bug in iPadOS15 that impacts markup/cropping of screenshots and Photos. Until the underlying issue is resolved, there are few 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.
Anyone that is experiencing these issues 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.