The belief that Man A was the father was likely strongly encoded and ingrained, given the emotional and practical significance of believing he was the father. This makes the belief impossibly resistant to being unbelieved or erased.
Consider the example of someone who strongly believes the earth is flat. They have held this belief for a long time and it is deeply ingrained in their view. Then, they are presented with overwhelming scientific evidence that the earth is actually spherical. They are shown satellite images, explained the concepts of gravity and curvature, and provided irrefutable proof that their flat earth belief is incorrect.
Even after accepting this new information and consciously changing their belief to align with the reality of the earths spherical shape, the original flat earth belief does not simply disappear from their mind.
Just as the mother in the paternity test example may still have remnants of the original belief about the father, the former flat earther would still have some cognitive residue of that initial belief, even after fully accepting the reality of the spherical earth.
The key point is that beliefs, once conceptualized, do not simply vanish when contradicted. They have everlasting presence in a persons mind and when shared with other minds they have the capacity to transcend the original idea holder and exist for generations.