Hello,
This one seems to be somewhat aleatory.
I decided to remove the second instrument in a few players’ instances (for example removing a player’s clarinet where supposed to play both clarinet and bass clarinet).
Back to writing, Dorico would allow me to input only a mid C whatever the staff, exactly as in a 1-line staff like, say, bass drum (try to figure out an 80-bass drum orchestra btw 
).
So I saved my score, quit Dorico and launched it back : I resumed writing my score normally.
I remember another strange behaviour when doing a similar operation : after removing some instruments in one or two players’ intances the order of the instruments had changed in the score and the note input behaved as if it was in the wrong staff.
That behaviour is erratic, though.
Any similar experience ?
Michaël