I use GPO4 as a plugin for Sibelius 6. A few weeks ago I noticed that some dynamics marking in scores were not playing back correctly. I reported this to Sibelius, and was eventually told:
I asked one of our developers to look into this, and I'm afraid this looks very much like a bug in GPO4. It seems as though GPO is unreliable in volume when switching legato on and off – remove the slur and the problem goes away. Is this a known problem? Something causing CC#1 to be ignored or misinterpreted? I'm pretty new to Sibelius but so far I haven't seen them point fingers at other products.
If this is a know problem, does it involve just legato? I found another place that did not involve a slur (which gets translated into legato control plus overlapping notes) but did involve just the opposite - staccato. I can no longer locate that one - I may have accidentally fixed it - but I'm pretty sure it was for a KS solo string of some sort. Not much in common with the 2 problems except "return to normal" - the CC#1 problem happens following some slurs (not all) and some staccatos (again, not all).