Tracker_Session & Tracker_Cookies record entries, but doesn't log user_id

After upgrading from 5.8 to 6.1 I had a bunch of errors with the Tracker.

Most of it some integrity constraints, but currently it doesn't log the user_id even if signed in.

Any idea how to fix this?

Start by reproducing a request while signed in and inspect how Tracker_Session and Tracker_Cookies record entries after the 5.8-to-6.1 upgrade, including the reported integrity-constraint errors. Done means signed-in requests store the expected user_id in the relevant records without violating constraints.

Written by the indexing model from the issue text.