All praise be to Allāh and may the peace and blessings be upon the Prophet Muhammad.
Using the 18-degree calculation is not one of Ijtihād in which imāms and committees can decide to choose one opinion over the other.
The conflicting position, which corresponds to a later fast starting time, has only been presented recently by a small number of groups and lone individuals.
If Imāms or committees still believe that the second opinion (later Fajr start time) is correct then it is obligatory for them to at least inform those under their influence of the 18-degree timetable thus allowing them the choice to investigate the matter further.
Personal worship vs. Communal matters
“O you who have believed, obey Allāh and obey the Messenger and those in authority among you. And if you disagree over anything, refer it to Allāh and the Messenger, if you should believe in Allāh and the Last Day. That is the best [way] and best in result”.
Sources: http://www.islam21c.com originally posted on 25th July 2012
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 under the chapter, ‘224: The Difference between the Principle of Fatwa and the principle of Judicial Judgement.’ Ibn Taymiyyah also elaborated on this point thoroughly.