For any extension that has a WebJar or UI module that depends on a platform WebJar, the version of the platform WebJar remains cached after upgrading XWiki, if the extension version doesn't change.
If the extension has a JSX / SSX then they will be cached, having the page version (or the extension version) in the URL.
If the extension has a WebJar resource, it will be cached having the extension version in the URL.
The cache of these extension resources doesn't get invalidated when upgrading XWiki if the extension version doesn't change, because the extension code doesn't change. The consequence is that if one of these resources depends on a platform WebJar resource, then the platform version is not updated when upgrading XWiki. The extension ends up requesting the previous version of the platform WebJar resource.
In order to fix this we need to include the platform version in the URL of JSX / SSX or WebJar resources that are evaluated, because any (Velocity) evaluation can potentially depend on the platform version.