Looking at the underlying study, this isn’t evidence of bias. It’s evidence of correlation between Republicans and negative sentiment.

If you look at the sentiment for public figures given, the bottom one is, for example, Brett Kavanaugh. Well, he was credibly accused of sexual assault during his confirmation hearings, which was a huge deal at the time. Someone with that on their record will probably be read as negative, but, I mean, not the editors’ fault!