Does the fast of one who does something during the day in Ramadan, thinking it breaks the fast, and intends to break the fast, knowing that what he did is forbidden but proceeds with it, then it becomes clear that his action was not among the things that break the fast, become void?

Whoever does what he believes breaks the fast, when in reality it does not, but proceeds with the belief that it will break his fast, is considered to have broken the fast. This is because he has abandoned the intention of fasting and rejected it, which invalidates the fast according to those who say that rejecting the intention invalidates it. Therefore, he must make up for that day; for intention is a pillar of fasting, and its corruption corrupts it, as stated in the Hadith: "Indeed, actions are by intentions."