Why does Allah increase the sickness of doubt in the hearts of hypocrites, even though this might contradict the principle of justice in providing equal opportunities for faith?

It is Allah Almighty's way to punish the one who turns away by increasing their turning away and prolonging their transgression, and to reward the guided one by increasing their guidance. This is part of His perfect justice and wisdom. The sickness in the hearts of the hypocrites is doubt, and Allah increases them in defilement, evil, and misguidance because they first preferred misguidance. It is also said that their sickness increases due to the triumph of the Prophet's message and their denial of the signs, or that it is a prayer against them. And Allah knows best.