Mixpanel's JavaScript SDK sets $search_engine by detecting the referring domain and matching it against a list of known search engine domains (e.g., google.com, bing.com). It also captures mp_keyword if search keywords are present in the referrer URL. [Default Event Properties]
However, $referrer and $referring_domain are session-level properties that can be absent in certain situations:
"Might not be present if document.referrer does not return a value. As opposed to $initial_referrer and $initial_referring_domain, this property will be omitted if the user lands directly or the referring website adds the rel="noreferrer" parameter." [Default Event Properties]
So what likely happened:
The first page view in the session captured the referrer from a search engine, allowing $search_engine to be set (and possibly stored as a super property or in the cookie).
Subsequent page views in the same session may no longer have document.referrer set (since the user is now navigating within your site), so $referrer and $referring_domain are absent.
UTM parameters (utm_source, utm_medium) are simply not present because the user arrived organically — no UTM tags were in the URL.
