Whenever the Powered Melee button is input with thunderclap the melee acts as an Auto Melee. This makes thunderclap incredibly difficult to use at close range as a uncharged melee will be used even with the use of the powered melee binding.
Heres a few posts detailing the same issue from Reddit.
https://www.reddit.com/r/DestinyTheGame/comments/wx20vs/normal_melee_is_prioritized_even_when_using_the/?utm_source=share&utm_medium=ios_app&utm_name=iossmf
https://www.reddit.com/r/DestinyTheGame/comments/wwnpw8/not_being_able_to_use_the_thunderclap_melee_while/?utm_source=share&utm_medium=ios_app&utm_name=iossmf
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Having this issue too