could you send a screenshot of the problem? That would be super helpful
Since he's just singing vowels it makes me think this is a preutterance/overlap or envelope issue...??
Honestly that'd be my first guess but since no screenshot it's a wild one haha Try selecting all the notes, hit reset envelope button, and p2p3 (or p2p4). Then while all the note are still selected right click and go to properties, and make sure the preutterance and overlap are white and not greyed out. If they're grey double click inside the box to reset them. (also check STP that it's white/reset too.) Not sure if that'll fix it completely because I'm not exactly sure that the exact issue is buuuut !!! the above is just always a good routine to do before tuning any UST.
And about him sounding whisper-y, did you accidentally download a whisper append instead of something else? (I've never used Shuu before so I'm not familiar with his appeneds sryyyy). Other than that maybe his voicebank has built in appends, meaning you have to use a certain alias on the note to call other appends (in other words some voicebanks need "↑" or a special symbol in the suffix to call the power append). Or it could just be the notes he's singing are too low and using one of his lower/softer pitches that were recorded. His higher pitches probably have a lot more power to them.
