What is the ruling regarding prayers previously combined based on an incorrect fatwa concerning the combining of prayers while traveling, and is it necessary to inform those to whom the fatwa was conveyed of its error?

Scholars differed concerning the distance that permits concessions during travel. Some, like Shaykh al-Islam Ibn Taymiyyah and Allamah Ibn Uthaymeen, held that it depends on common practice ( عرف ), so whatever is considered travel by people's custom allows for concessions. There are other opinions that specify a shorter distance, such as what was narrated from Ibn Umar and Ali (may Allah be pleased with them), and Dihyah al-Kalbi and Anas ibn Malik (may Allah be pleased with them). Although the adopted fatwa may differ, it is permissible to follow someone who gives a fatwa according to the other opinion, especially if it is after the act has occurred and it is difficult to rectify. Therefore, you do not need to repeat the prayers you combined and shortened.