If I do a UDM Search for network.email.to = "<emailaddr>" it returns a result, but if I copy the UDM for network.session_id = "<sessionid>" for that result and search on that, it returns no results for the same time window:

The log was ingested ~41 hours ago, circa 2023-10-02 at 9PM Pacific and I'm searching for it on 2023-10-04 at 2PM Pacific, but perhaps it still hasn't indexed that field for searching?