Uploading Musescore 4 piece to Musescore

I have a Musecore 4 composition that is choral SSAATTBB piece. I am trying to set up video learning tracks (dominant parts with a Grand Piano as the accented voice with the other voices slightly muted). This sounds perfect in Musescore 4 (and exported mp3s sound fine), but when I upload it to Musescore (and onwards to Youtube), the sounds and volumes that were set up in the score disappear (all voices show as Choir Ahs, full volume). Am I missing something, or is this a bug?

musescore.com/user/28096429/scores/10569568/s/zHhRKK?share=copy…

P.S. I am new to Musescore 4 and only loaded it to circumvent the fact that uploading to Youtube appears to be broken for Musescore 3!