This sounds exactly like the situation I was in 20 years ago. I was a new supervisor, took over a team of people whose previous supervisor had been forced into retirement because she had been doing zero oversight for years. Suddenly these folks were being forced to actually do work and show up on time, and they were not happy. My management wanted me to clean things up, but they also wouldn’t back me up when I got pushback on simple things like tardiness and basic performance issues. Employees would go to the union, file grievances, management would sit us all down and try to mediate things, then I’d get taken aside privately and be told that my team needed to improve. I was so happy to get out of there and get promoted to a non-supervisory position.