Answering questions in the positive is a simple kind of bias that basically all LLMs have. Frankly if you are going to train on human data you will see this bias because its everywhere.

LLMs have another related bias though, which is a bit more subtle and easy to trip up on, which is that if you give options A or B, and then reorder it so it is B or A, the result may change. And I don't mean change randomly the distribution of the outcomes will likely change significantly.