The Scientific Method includes a step in which you state your Hypothesis - an educated guess, based on information you already know. There is nothing wrong with that, because it means you are already familiar the established science.

The issue comes when the experiment uncovers unexpected data and/or conclusions. The proper scientific response is to adjust, or even reject, the hypothesis based on the new data. Someone with good Critical Thinking Skills would have no problem doing that, because a subjective approach, coming up with a truthful conclusion, supported by the data, is always the objective.

Unfortunately, too many people have a personal desire to make their original hypothesis the truth, either because of their ego, or because they have some sort of personal or economic investment in that hypothesis, etc. These are people who are only using the promise of Critical Thinking to add credibility to their conclusions, when in reality, they were always looking to confirm their own bias.

And sometimes the research DOES confirm your hypothesis. That’s not necessarily confirmation bias, as long as your hypothesis was always based on accepted scientific principles. Scientists often have a pretty good idea of the outcome of an experiment. A person looking for confirmation bias goes into an experiment hoping to prove their hypothesis correct, while a true scientist goes in hoping that something unexpected will happen, because that gives them something new and interesting to study.