That would be my expectation, yes.
Of course, I cannot judge if this is in intended behavior. For all I know it could be written in such a way it only uses the user_id field to correctly pick up on existing sessions upon plugin activation and it adds it to the data field after that.
I presume, however, this must indeed be a bug simply because the user_id field is what it appears to show in the session list in the dashboard.
So in my case all listed sessions always show user ID 0. Every single one… even though I’m looking at that list in the dashboard for which I need to be logged in. It seems a little paradoxical.