Is refraining from issuing a fatwa on controversial issues—when the questioner is already acting upon one of the opinions, yet is unaware of all the opinions and their correctness—considered a concealment of knowledge?

May Allah reward your piety for refraining from issuing fatwas due to their seriousness. However, strive to seek knowledge so that you may have insight, for indeed, those who teach people good are prayed for by the inhabitants of the heavens and the earth. If you are a follower (muqallid) of someone whose opinion among the people of knowledge is trusted, then there is no harm in you issuing a fatwa based on their قول (saying/opinion), attributing it to them. And it may be more appropriate to issue a fatwa based on taqleed (following a scholar) if you fear that the questioner will follow their desires or seek a fatwa from those who are not qualified. The sayings of scholars regarding issuing fatwas based on taqleed are three: absolute prohibition, permissibility only for oneself, and permissibility when there is a need and no mujtahid (independent jurist) is available, which is the most correct view. As for a muqallid seeking a fatwa, if a scholar is found who can guide them to the right path, it is not permissible for them to seek a fatwa from another muqallid. But if no one else is available, then it is permissible.