A similar effect is seen on his-tag purification when the b-me reacts with nickel ions bound to the column so it might be possible that your few samples have unusually high levels of ne of the transition metal ions in their serum/plasma leading to precipitatin of metal sulphides

Mesure the pH of the protein solution. In the acidic pH the color of coomassie blue or bromophenol blue turns brown.

also in my experiments high ionic buffer may change the color of sample.