Only affects built-in Symfony reverse proxy, aka HttpCache
A BC break has been introduced in FOSHttpCacheBundle 1.2.0:
BC break: If you overwrite cleanupForwardRequest in your cache kernel, you need to extend FOS\HttpCache\SymfonyCache\UserContextSubscriber and move that logic to the method cleanupHashLookupRequest in there.
cleanupHashLookupRequest() method was initially located in base HttpCache class and was overridden by eZ implementation to embed original request (for siteaccess matching). Original location being now absent, cache is reacting erratically, i.e. with user context hash cache.
This can cause wrongly shared Http caches.