Which java are you currently using? OpenJDK has some known issues and Zulu is the main recommendation. I don’t know if this is one of the possibly issues, but it’s worth checking.
Other possible things to test:
Have you defined other default standalone widgets for other item types? It also seems possible that the UI is having some sort of trouble with the stream. If there’s an error in one of those widgets it may be interfering with the UI’s ability to render the item page in general?
Have you tried clearing your browser cache? There have also been some strange UI effects reported with some caching issues.