Adat 1+2 in: Daw Mix (coming from an RME Fireface). These two channels are active in mixer 1 (without any effects). Mixer 1 is routet to outputs 1+2. The reverb on/off switch is off in that mixer.
Adat 3 in: Vocal microphone (coming from an RME Fireface). This channel is active in mixer 2 and and the reverb send is active. Mixer 2 is routet to outputs 3+4. The Reverb switch is on in that mixer.
Now: why do the outputs 1+2 contain the reverb from the vocals ?