Fixers need good air cover
In the right sort of scenario, both management and the employee have an understanding about how things will work. Management sends the employee into the most messed up situation by definition, since that's where the biggest gains can be had. It's the place that needs them most.
The employee reports back, and starts recommending changes, and if necessary, starts making them. This is particularly important if it's an active problem spot, where every day is costing you reliability, or money, or customers, or whatever your metric is.
Odds are, the fixer is going to find some technical problems. Those problems will need to be resolved, and solutions will be proposed. When this happens, the team can either be responsive or not. If the team is not responsive, it's on management to do something about it. The fixer has no true power to wield, no matter how much management talks about having to practice "leadership" and "influence". If the team won't budge, it's on the people with the power to hire and fire. The fixer is just another non-management employee.
After working on things for long enough, and having enough attempts to work with a team go sideways, the problem will become clear: it's the membership of the team and/or the management. It's actually usually both, since the membership of the team is a function of ... that's right, the management. This will be reported back up the chain, and it's expected that things will happen.
More than anything else, one key thing is expected: that the fixer will receive proper "air cover", and management will understand that the feedback which comes in from the broken teams, employees, and yes, managers is bound to be heavily biased against the fixer. After all, who exposed the problems? Who proved that the problem was in fact those individuals? You'd expect them to bring out the knives and aim them all at the fixer.
Basically, I realized too late that it had happened again.
When the air cover isn't there, the feedback starts going to very strange places. They may try to find problems with the technical angle the fixer pursued. But, let's say the fixer has the technical high ground for whatever reason -- skill, luck, phase of the moon, you get the idea. What's left?
That's right, you go for the soft skills. You start talking about the way they talk, or the way they run a meeting, or what they mention in meetings, or what they write about. This is all subjective, so there's no way to really push back on such a thing and make it stick.
Now, for the fixer, that really hurts. It's feedback which goes beyond them as an employee and targets them as an individual. It's striking at the part of them which exists all day every day, and doesn't "shut down" when they leave the office or close the laptop.
Guess what happens next.