1/2. Are you letting Sibelius auto-load patches for you? Or are you trying to do that yourself? There is an exception, but for the most part, everything in the Mixer should be set to (Auto).
2. Are you using a MIDI keyboard for input? Without looking deeper, sounds like that instrument's percussion mapping has become corrupted. Is this a file you've been working in for a while?
2. Are you using a MIDI keyboard for input? Without looking deeper, sounds like that instrument's percussion mapping has become corrupted. Is this a file you've been working in for a while?