People go out of their way to be mean to fat people. The fat acceptance movement is essentially “please don’t do that”.
Every philosophy or movement has outliers that misinterpret and act outside of the bounds of that movement and we understand that those outliers don’t represent the group; yet people always look to those outliers with the fat acceptance movement — the fat acceptance movement is not given the benefit of the doubt like others are.
To me that implies that there is underlying bias.
From my vantage point the movement has largely been co-opted by extremists who want to pretend that obesity is not a health problem.
(It's does not. It made me so much worse. I have nothing but bad things to say about the medical "professional" who catastrophized my diagnosis to "wake me up" and then shrugged when my response was depressed resignation.)
Specifically, doctors are less likely to investigate issues that can be explained by obesity than they are in non-fat people.
For example, if you have high blood pressure and you are a normal weight your doctor will probably look into underlying conditions that can cause high blood pressure. Being overweight is an independent risk factor for high blood pressure but it doesn’t reduce the likelihood of the other risk factors. Doctors are less likely to look into an underlying cause for high blood pressure if you are overweight.
In general doctors are looking for the most obvious potential causes of a health problem. If someone on high dose anabolic steroids went to the doctor with him blood pressure, the doctor would probably tell them to cut out the steroids as the obvious answer rather than exploring other reasons the patient’s blood pressure might be high.