The Garritan GPO doc (A Guide to Garritan PERSONAL ORCHESTRA®
Version 4) has several references to "Legato Controller CC# 64".
The MIDI standard (if I read it correctly) says CC# 64 is "Sustain"; "legato" is CC# 68. Sibelius support says you use CC# 68.
This wouldn't be a big deal except that Sibelius is apparently turning on and off CC# 68 for you. Is this doing anything to/for GPO? I don't see CC# 68 mentioned in the doc at all,but I could have missed it.