That’s because everyone in the general public was forced into the office before the government workers. At least the majority of them. If the government unions were fighting a return to office for everyone else, then there would be solidarity going back to fight it for the government office workers. Instead everyone just feels that it’s a good thing the gov’t office workers are going back because hey, my employer made me go back, why should they have it any better?
A gain for unions might be a gain for non union if we’re talking comparable industries, like union electricians or plumbers. Those are industries where people can leave their non union employer and go union, or work to unionize their employer.
For general office work that’s not the case. Government administration positions don’t all have non union equivalents. Office workers don’t generally unionize. You can’t just quit your non union job and get a government job, those are hard to come by. So a gain for government service workers is irrelevant to the general public, or in the case of increased wages just means increased taxes. There’s a sense of jealousy of those with government positions, as well as schadenfreude when they get screwed over.