One potential reason you could look into is this: Disable cache on active sessions only · Issue #3976 · getkirby/kirby · GitHub – this affects the page cache both in case you are logged in or any template/snippet/hook/plugin etc. that is called for the request creates or checks for a session.
Whether this is the reason in your case can be determined by looking for a cache-control: no-store response header for the page request (the HTML document), which is returned in this case.