EDIT: "Kamala Harris is more liberal than Bernie Sanders"
https://www.newsweek.com/kamala-harris-more-liberal-bernie-sanders-senate-record-analysis-shows-1524481
Looks to be because Harris doesn't go along on bipartisan bills, and because none of Sander's bills ever become law.
As much as I like GovTrack that's a pretty crummy indicator, to be honest. (Their methodology and whatnot is
here.) Though they do keep it to a limited time period, meaning that Sanders's longer political career doesn't affect things as much, these cosponsorship figures don't account for things like actual bill content. It also glosses over plenty of other activity that goes on in congress that aren't necessarily tracked as easily, such as hearing activity, what they choose to amplify via press releases or other public activity, endorsements, etc. etc. (The lack of voting records isn't really a problem in the current Senate, as McConnell has let
very little legislation reach the floor this congress.)
What's more, going to that link shows that over a larger time period Harris is no longer the 'most liberal' member of the Senate by their reckoning. (She becomes #4.)
(GovTrack's website currently
has some caveats up about people using that takeaway, too.)
I'd say that data suggests that she's been highly party loyal recently, though, which is a separately notable figure.