My synth audio ouput is linked on the audiobox inst 1 channel
and this synth audio ouput is assigned to an audio entry in S1

When the channel volume slider is fully down in S1, I still can hear my synth audio coming out the audiobox and when I push up the mixer chanel volume slider, I can hear a mix of the direct audio signal coming out of the audiobox and the one comming out of S1.
The result is a phasey sound.
The mixer knob on the audiobox is fully turned to the right (playback) so I should not hear the direct audio sound ?

How can I avoid that direct sound coming out of the audio box ?

Note : I red the manual which explained that :
"Power User Tip: When monitoring your inputs through Studio One or another DAW, it is important that you turn the Mixer knob all the way to the Playback position. Monitoring both the input signal and the playback stream will create a doubling effect that will make monitoring dificult"
So why does the audiobox creates a doubling effect when the mixer knob is all the way to the playback position ?
Is the audiobox damaged ?