Now this is strange! I didn't even switch it off or on again, I simply pressed the MIDI button to check on the 'local control' (incidentally it was set to 'on') as per your suggestion.
And suddenly the missing voices returned! How bizarre.
I have recently been using the D50v software programmer (finally I'm able to get to grips with programming the D-50) so maybe all the MIDI back n forth was messing something up somewhere. Seems strange that the problem was cured just by pressing the 'MIDI' button though.
With regard to the D50 Librarian echoing the incoming MIDI back to the D-50, yes it's a problem which someone spotted recently. It was never an issue for me because I was using a D-550!!! What is happening is MIDI channel data which is coming in is being fed back out - it should only be doing this if the incoming MIDI channel data is coming from the "Remote Keyboard" input.