The log volume isn't too bad, but it doesn't seem particularly useful. And, perhaps more importantly, it seems worrying that possibly one of the most common ways to start a session (visiting the login page) is precieved by the code as an "unknown reason", which suggets there may be a bug in the code of unknown severity.

https://logstash.wikimedia.org/goto/029a81c3dbd7410c10a3ea3aad87da8d

Source code: https://gerrit.wikimedia.org/g/mediawiki/core/+/35098e1763c7e8917415ff55bc5a3e3c4815b128/includes/session/SessionBackend.php#829

Reproducible by visiting https://meta.wikimedia.org/wiki/Special:UserLogin in private browsing, including with any other wiki hostname.