Nothing to worry about too much but I ran into this and spent some time to understand what's going on.
Maybe it saves some time for someone running into the same problem.
As the project is updating to the new Jasper Version 7 and the master branch including the examples is already on it but the official realeasd docker images are not we MUST NOT use examples from master branch with the official docker images.
It seems the xml format has changed so that the new XML format does not anymore validate with the old schemas.
Use the git tag that corresponds to your installation in order to get compatible examples or use the Docker tag master in order to use the latest version as the examples from master branch work.
Reference to the commit bumping the version: bdad37f
Maybe a maintainer could add a word about the timeline for a release containing the update to 7.x?