I can not say exactly whats going on, i don't have this all in my head and also I don't know what your interface is.
In general return can only bring something that is send to.
is there a send not fully closed ?
Soloing can be pre or post fader ?
Also possible the DAW is providing unexpected signal somewhere ?
This is not a solution for your problem, I know, it's just some thoughts if you allow me.
edit :
I had a short look into the user manual and it confirms the solo ist post fader.
And the send-FX is coupled to the fader.
Soloing the channel does not mute the sendFX or returnFX.
During a live show you can solo an input channel for adjusting EQ or dynamics without disturbing the reverb, this makes sense to me. Imagine the live concert and the Reverb goes on and off because you listening some talents solo. For your headphone just close the return on the headphone output channel.
M1-Sequoia, Madiface Pro, Digiface USB, Babyface silver and blue