Just my thoughts on this:
I have to agree with the game mechanism.
I think it takes an ammount of concentration to set a trap properly. While concentrating on setting the trap you would theoretically not be focused on hiding/ remaining unseen.
Rp wise i could see some instances where it could be possible to remain hidden while setting a trap but in the vast majority of situations i think you'd be more focused on doing one or the other.
Also as ring said :
I think it may be because the game considers it a hostile action.

I don't know if this is hard coded but i'd guess it is and wouldn't be able to be fixed easily..

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