Before charcoal can be used it must soak in nutrients for a longer time. If not it will be a detriment to any plant.
No ,its not detrimental to any plant.Charcoal its inert,doeant decompose too easy and doesnt decompose by bacteria.
I use fresh charcoal on hundreds of trees in my orchard and i do add only nitrogen right before i use it.
The reason why your citrus struggled with charcoal its not because of the charcoal itself but because of the ash.
Charcoal is not ash.
Ash its alkaline and your citrus want acidic soil not alkaline so in order to use charcoal for citrus or otther acid loving plants you have to wash it and make sure has as little ash as possible.
Also use max 8 percent charcoal for citrus.For my orchard trees i use 25 percent but because charcoal contains somme ash ,for citrus you have to use less.