Hello Phil!
There is a playback bug in Sibelius, which sometimes occurs when the same instrument is used more than once. Normally, it helps to create the problematic instrument once again, cut the content of the old instrument and paste it into the new one.
If this doesn't help, try the following thing: Go to the Sibelius mixer and reassign the sound ID (third line in the mixer) of the instrument, that doesn't switch correctly. If you use the same sound ID more than once, be careful to assign it in the right order. For example the first line with "strings.violin" for the first violin and the second line with "strings.violin" for the second violin.
Best,
Andi
Vienna Symphonic Library