Thanks
@Matt Bartlett @Andreeas1978
Mitch from Chord support also advised me to simply set a maximum comfortable level on Mojo in two use cases: IEMs and headphones. That seems to be the very best way to deal with this and preserve one's hearing from serious damage.
I am also curious as to what would be to blame for the behaviour I witnessed yesterday: sound going up by a lot on its own and distortion for about 10 seconds, followed by everything going back down to normal.
I was thinking of simply swapping Poly for a new unit but this would not change anything if it is the Mojo and not Poly.