It sounded like the whole work culture there brainwashes them into hating Lienna anyways, so the older ones were probably already fully converted by virtue of still being there after so long.
They don't even need to do brainwashing. Assumptions and ignorance is all you need. I mean most knights didn't see Lienna on the front lines as much once she got promoted, so most people assume she's living the cushy life, this making her weak and soft. Couple that with the fact that she is a woman, misogyny is a powerful tool of hate and disdain. No need to even spread rumors at that point.
You even see that IRL.
I've worked with a manager who was hated and a lot of the staff shit talked him behind his back. He wasnt the best people person and we barely got to see him during our shift so it was an easy assumption to think all the bad things. Once I decided to talk to him during a lunch break, I found out how much actual shit he actually has to deal with, not only from corporate but from other managers and staff who want his position or don't like him to begin with.
He was trying his hardest to deliver a softer punch that he was getting from corporate.
Soon enough once he actually left and one of the other managers got his spot, everyone slowly started to see how much he did.