I made a piano sketch and imported the midi file to Dorico to work it out. The piano sketch is a two stave system (not edited, just a split point between bass and treble) which is ok for working. From there I copy fragments or longer parts into other instruments.
When I copy mixed parts from the upper and lower staves from the piano Dorico adds two different voices into the destination instrument (stems down from bass clef and stems up from treble) which doesn’t make sense if it’s a monophonic instrument. Sure it’s the “right” way of dealing with it… but I have to bring back the lower voice notes to the upper one very often. Is it possible to keep instruments in one-voice-only-mode or is there a “paste everything to voice one only” command?
Another option is to select your material after pasting and right click > voices > change voice. Alternatively you can “paste into voice” to avoid the extra step.
Regarding how it is pasting, it’s a bit of a tall order to not expect it to create two voices when you are condensing down to an instrument that isn’t a grand-staff. It is perfectly logical to me that it would do that in an effort to avoid overwriting one part or the other. This allows the pasting to preserve the data from each stave. If it can exist all on one stave to begin with, perhaps you should adjust the piano part first, by putting everything in the treble clef and then merging into one voice, after that copy/pasting will be a breeze.