The browsers don't share third-party caches across domains, because it made it easier to leak information about which sites the user has visited.
Do you have a source for this? I believe(d?) it too, but when re-checking just now, I could not find any.
Safari: https://webkit.org/blog/8613/intelligent-tracking-prevention...
Chrome: https://developers.google.com/web/updates/2020/10/http-cache...
Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1536058