Hello and welcome to the forum!
Please make sure to choose the right Sibelius instruments for the strings. Choose Solo Violin, Solo Viola, ... for solo strings and Violin 1, Violin 2, Violin I, Viola, ... for ensemble strings. Also make sure to choose the right programs on the Manual Sound Sets page (solo or ensemble).
If you confuse these settings it will lead to strange playback behaviour.
By the way, if you haven't done so yet, I recommend reading the "Optimizing Sibelius Playback" manual you find at the following location:
http://vsl.co.at/en/68/141/460/576/655.htm
Best regards,
Andi Olszewski
Vienna Symphonic Library
Vienna Symphonic Library