Instrument Voices not being Preserved in Save Files

In many of the pieces I am currently working on, as well as any I download from this website, the instrument parts are named correctly, but playback will not keep the instruments' sounds. If it is the first tab I open, all sounds will be played back as piano. If opened after another tab, where the voices are correctly playing, it will copy those voices before shifting to various midi instruments (eg; I opened a choral piece, fixed the sounds, then opened a Pierrot ensemble to find that the first four lines were choir, then percussion instead of violin, then trombone instead of cello). When exported to .mp3, all sounds are correct.

The problem goes away when I change the instrument on a part, including changing it to the same instrument (in Staff/Part Properties). However, repeatedly doing this is time consuming and inefficient.

I don't know quite how to attach a score for this, as it appears to function for any score I open, even those which others have had no instrumentation issues with.