I have a fresh installation of JDeveloper 126.96.36.199.0 on Windows 11 (previously had Win10 and faced same issue).
When I create a new OSB application and project, shut down JDeveloper and later reopen it, the composite isn't displayed correctly; it is either blank or shows the business connectors only.
I have to close and reopen JDeveloper multiple times to get it to display. Just closing and opening the project or application doesn't help.
There is a similar issue reported on https://community.oracle.com/tech/apps-infra/discussion/4284885/12c-jdeveloper-osb-composite-xml-is-not-loading-or-not-showing-any-flow-design but that is for a project migrated from 11G. Besides, my JPR file doesn't have that element.
I have downloaded and installed the latest available patch 33958532.