There does appear to be a bug affecting the Windowed Apps display mode of iPadOS 26. Resizing and moving the affected App window appears to resolve the display glitch.
You and others that encounter this issue would be well advised to submit a Bug Report to Apple via its Product Feedback portal. For iPad and iPadOS:
Feedback - iPad - Apple
Every report counts - as until Apple receive a sufficient number or reports, it is perhaps unlikely that resources will be assigned to investigate and resolve the issue.
When submitting a report, ensure that you include as much detail as possible within the body of the report - including your iPad model and installed version of iPadOS; don’t rely upon the pick-list, as this rarely includes the full range of device models or most recent versions of iOS/iPadOS.