Hello,
I found a bug with /stopsound command.

If you play a specific sound in the master source (/playsound entity.enderdragon.ambient master @a), you can't stop it with sound-specific /stopsound command.

Running /playsound entity.enderdragon.ambient master @a
Then the command /stopsound @a master entity.enderdragon.ambient doesn't stop sound, even though I get a response telling me that the sound was stopped.
/stopsound @a master doesn't stop the whole group either.
Finally /stopsound @a master stops all sounds.

Other sound sources, like player, ambient and others, work fine.

I tried playing the default Minecraft sounds as well as Resource pack specific sounds which are defined in sounds.json.